AArch64: Fix error checking for SIMD udot (by element)

Committed on behalf of Matthew Malcomson:

The SIMD UDOT instruction assembly has an unusual operand that selects a single
32 bit element with the mnemonic 4B.
This unusual mnemonic is handled by a special operand qualifier and associated
qualifier data in `aarch64_opnd_qualifiers`.

The current qualifier data describes 4 1-byte elements with the structure
{1, 4, 0x0, "4b", OQK_OPD_VARIANT}
This makes sense, as the instruction does work on 4 1-byte elements, however
some logic in the `operand_general_constraint_met_p` makes assumptions about
the range of index allowed when selecting a SIMD_ELEMENT depending on element
size.
That function reasons that e.g. in order to select a byte-sized element in a 16
byte V register an index must allow selection of one of the 16 elements and
hence its range will be in [0,15].

This reasoning breaks with the above description of a 4 part selection of 1
byte elements and allows an index outside the valid [0,3] range, triggering an
assert later on in the program in `aarch64_ins_reglane`.

vshcmd: > echo 'udot v0.2s, v1.8b, v2.4b[4]' | ../src/binutils-build/gas/as-new -march=armv8.4-a
as-new: ../../binutils-gdb/opcodes/aarch64-asm.c:134: aarch64_ins_reglane: Assertion `reglane_index < 4' failed.
{standard input}: Assembler messages:
{standard input}:1: Internal error (Aborted).
Please report this bug.

This patch changes the operand qualifier data so that it describes a single
32 bit element.
{4, 1, 0x0, "4b", OQK_OPD_VARIANT}
Hence the calculation in `operand_general_constraint_met_p` provides the
correct answer and the usual error checking machinery is used.

vshcmd: > echo 'udot v0.2s, v1.8b, v2.4b[4]' | ../src/binutils-build/gas/as-new -march=armv8.4-a
{standard input}: Assembler messages:
{standard input}:1: Error: register element index out of range 0 to 3 at operand 3 -- `udot v0.2s,v1.8b,v2.4b[4]'

4 files changed, 28 insertions, 0 deletions

diff --git a/gas/ChangeLog b/gas/ChangeLog
index 69187bb..7ba30cb 100644
--- a/gas/ChangeLog
+++ b/gas/ChangeLog
@@ -1,3 +1,10 @@
+2018-10-16  Matthew Malcomson  <matthew.malcomson@arm.com>
+
+	* testsuite/gas/aarch64/illegal-dotproduct.d: New test.
+	* testsuite/gas/aarch64/illegal-dotproduct.l: New test.
+	* testsuite/gas/aarch64/illegal-dotproduct.s: New test.
+
+
 2018-10-15  Alan Modra  <amodra@gmail.com>
 
 	PR 23534
diff --git a/gas/testsuite/gas/aarch64/illegal-dotproduct.d b/gas/testsuite/gas/aarch64/illegal-dotproduct.d
new file mode 100644
index 0000000..3f8928d
--- /dev/null
+++ b/gas/testsuite/gas/aarch64/illegal-dotproduct.d
@@ -0,0 +1,4 @@
+#as: -march=armv8.2-a+dotprod
+#name: Invalid dotproduct instructions.
+#source: illegal-dotproduct.s
+#error_output: illegal-dotproduct.l
diff --git a/gas/testsuite/gas/aarch64/illegal-dotproduct.l b/gas/testsuite/gas/aarch64/illegal-dotproduct.l
new file mode 100644
index 0000000..06d0d78
--- /dev/null
+++ b/gas/testsuite/gas/aarch64/illegal-dotproduct.l
@@ -0,0 +1,13 @@
+[^:]+: Assembler messages:
+[^:]+:[0-9]+: Error: register element index out of range 0 to 3 at operand 3 -- `udot V0.2S,V0.8B,V0.4B\[4\]'
+[^:]+:[0-9]+: Error: operand mismatch -- `udot V0.4S,V0.8B,V0.4B\[4\]'
+[^:]+:[0-9]+: Info:    did you mean this\?
+[^:]+:[0-9]+: Info:    	udot v0.2s, v0.8b, v0.4b\[4\]
+[^:]+:[0-9]+: Info:    other valid variant\(s\):
+[^:]+:[0-9]+: Info:    	udot v0.4s, v0.16b, v0.4b\[4\]
+[^:]+:[0-9]+: Error: register element index out of range 0 to 3 at operand 3 -- `sdot V0.2S,V0.8B,V0.4B\[4\]'
+[^:]+:[0-9]+: Error: operand mismatch -- `sdot V0.2S,V0.8B,V0.4H\[4\]'
+[^:]+:[0-9]+: Info:    did you mean this\?
+[^:]+:[0-9]+: Info:    	sdot v0.2s, v0.8b, v0.4b\[4\]
+[^:]+:[0-9]+: Info:    other valid variant\(s\):
+[^:]+:[0-9]+: Info:    	sdot v0.4s, v0.16b, v0.4b\[4\]
diff --git a/gas/testsuite/gas/aarch64/illegal-dotproduct.s b/gas/testsuite/gas/aarch64/illegal-dotproduct.s
new file mode 100644
index 0000000..9c714ae
--- /dev/null
+++ b/gas/testsuite/gas/aarch64/illegal-dotproduct.s
@@ -0,0 +1,4 @@
+UDOT	V0.2S, V0.8B, V0.4B[4]
+UDOT	V0.4S, V0.8B, V0.4B[4]
+SDOT	V0.2S, V0.8B, V0.4B[4]
+SDOT	V0.2S, V0.8B, V0.4H[4]