aarch64: Check for register aliases before mnemonics

Previously we would not accept:

	A .req B

if A happened to be the name of an instruction.  Adding new
instructions could therefore invalidate existing register aliases.

I noticed this with a test that used "zero" as a register alias
for "xzr", where "zero" is now also the name of an SME instruction.
I don't have any evidence that "real" code is doing this, but it
seems at least plausible.

This patch switches things so that we check for register aliases
first.  It might slow down parsing slightly, but the difference
is unlikely to be noticeable.

Things like:

	b	.req + 0

still work, since create_register_alias checks for " .req ",
and with the input scrubber, we'll only keep whitespace after
.req if it's followed by another name.  If there's some valid
expression that I haven't thought about that is scrubbed to
" .req ", users could avoid the ambiguity by wrapping .req
in parentheses.

The new test for invalid aliases already passed.  I just wanted
something to exercise the !dot condition.

I can't find a way of exercising the (existing) p == base condition,
but I'm not brave enough to say that it can never happen.  If it does
happen, get_mnemonic_name would return an empty string.

gas/
	* config/tc-aarch64.c (opcode_lookup): Move mnemonic extraction
	code to...
	(md_assemble): ...here.  Check for register aliases first.
	* testsuite/gas/aarch64/register_aliases.d,
	testsuite/gas/aarch64/register_aliases.s: Test for a register
	alias called "zero".
	* testsuite/gas/aarch64/register_aliases_invalid.d,
	testsuite/gas/aarch64/register_aliases_invalid.l,
	testsuite/gas/aarch64/register_aliases_invalid.s: New test.

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