aarch64: Fix MOVPRFX markup for bf16 conversions

aarch64: Fix MOVPRFX markup for bf16 conversions

bfcvt converts a .S input to a .H output, so any predicated movprfx
needs to operate on .S rather than .H.  In common with SVE2 narrowing
top operations, bfcvtnt doesn't accept movprfx.

2020-02-10  Richard Sandiford  <richard.sandiford@arm.com>

opcodes/
	* aarch64-tbl.h (aarch64_opcode): Set C_MAX_ELEM for SVE bfcvt.
	Remove C_SCAN_MOVPRFX for SVE bfcvtnt.

gas/
	* testsuite/gas/aarch64/sve-bfloat-movprfx.s: Use .h rather than
	.s for the movprfx.
	* testsuite/gas/aarch64/sve-bfloat-movprfx.d: Update accordingly.
	* testsuite/gas/aarch64/sve-movprfx_28.d,
	* testsuite/gas/aarch64/sve-movprfx_28.l,
	* testsuite/gas/aarch64/sve-movprfx_28.s: New test.