[AArch64][SVE 09/32] Improve error messages for invalid floats

[AArch64][SVE 09/32] Improve error messages for invalid floats

Previously:

        fmov d0, #2

would give an error:

        Operand 2 should be an integer register

whereas the user probably just forgot to add the ".0" to make:

        fmov d0, #2.0

This patch reports an invalid floating point constant unless the
operand is obviously a register.

The FPIMM8 handling is only relevant for SVE.  Without it:

        fmov z0, z1

would try to parse z1 as an integer immediate zero (the res2 path),
whereas it's more likely that the user forgot the predicate.  This is
tested by the final patch.

gas/
	* config/tc-aarch64.c (parse_aarch64_imm_float): Report a specific
	low-severity error for registers.
	(parse_operands): Report an invalid floating point constant for
	if parsing an FPIMM8 fails, and if no better error has been
	recorded.
	* testsuite/gas/aarch64/diagnostic.s,
	testsuite/gas/aarch64/diagnostic.l: Add tests for integer operands
	to FMOV.