Introduce smul_highpart and umul_highpart RTX for high-part multiplications

This patch introduces new RTX codes to allow the RTL passes and
backends to consistently represent high-part multiplications.
Currently, the RTL used by different backends for expanding
smul<mode>3_highpart and umul<mode>3_highpart varies greatly,
with many but not all choosing to express this something like:

(define_insn "smuldi3_highpart"
  [(set (match_operand:DI 0 "nvptx_register_operand" "=R")
       (truncate:DI
        (lshiftrt:TI
         (mult:TI (sign_extend:TI
                   (match_operand:DI 1 "nvptx_register_operand" "R"))
                  (sign_extend:TI
                   (match_operand:DI 2 "nvptx_register_operand" "R")))
         (const_int 64))))]
  ""
  "%.\\tmul.hi.s64\\t%0, %1, %2;")

One complication with using this "widening multiplication" representation
is that it requires an intermediate in a wider mode, making it difficult
or impossible to encode a high-part multiplication of the widest supported
integer mode.  A second is that it can interfere with optimization; for
example simplify-rtx.c contains the comment:

   case TRUNCATE:
      /* Don't optimize (lshiftrt (mult ...)) as it would interfere
         with the umulXi3_highpart patterns.  */

Hopefully these problems are solved (or reduced) by introducing a
new canonical form for high-part multiplications in RTL passes.
This also simplifies insn patterns when one operand is constant.

Whilst implementing some constant folding simplifications and
compile-time evaluation of these new RTX codes, I noticed that
this functionality could also be added for the existing saturating
arithmetic RTX codes.  Then likewise when documenting these new RTX
codes, I also took the opportunity to silence the @xref warnings in
invoke.texi.

2021-10-07  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* rtl.def (SMUL_HIGHPART, UMUL_HIGHPART): New RTX codes for
	representing signed and unsigned high-part multiplication resp.
	* simplify-rtx.c (simplify_binary_operation_1) [SMUL_HIGHPART,
	UMUL_HIGHPART]: Simplify high-part multiplications by zero.
	[SS_PLUS, US_PLUS, SS_MINUS, US_MINUS, SS_MULT, US_MULT,
	SS_DIV, US_DIV]: Similar simplifications for saturating
	arithmetic.
	(simplify_const_binary_operation) [SS_PLUS, US_PLUS, SS_MINUS,
	US_MINUS, SS_MULT, US_MULT, SMUL_HIGHPART, UMUL_HIGHPART]:
	Implement compile-time evaluation for constant operands.

	* dwarf2out.c (mem_loc_descriptor): Skip SMUL_HIGHPART and
	UMUL_HIGHPART.
	* doc/rtl.texi (smul_highpart, umul_highpart): Document RTX codes.
	* doc/md.texi (smul@var{m}3_highpart, umul@var{m3}_highpart):
	Mention the new smul_highpart and umul_highpart RTX codes.
	* doc/invoke.texi: Silence @xref "compilation" warnings.

gcc/testsuite/ChangeLog
	* gcc.target/i386/sse2-mmx-paddsb-2.c: New test case.
	* gcc.target/i386/sse2-mmx-paddusb-2.c: New test case.
	* gcc.target/i386/sse2-mmx-psubsb-2.c: New test case.
	* gcc.target/i386/sse2-mmx-psubusb-2.c: New test case.

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