[AArch64][SVE2] Asm: support Floating Point Widening Multiply-Add

[c-rhodes]

Summary

Patch adds support for the indexed and unpredicated vectors forms of the
FMLALB, FMLALT, FMLSLB and FMLSLT instructions.

The specification can be found here:
https://developer.arm.com/docs/ddi0602/latest

[chill]

test/MC/AArch64/SVE/movprfx-diagnostics.s

114 ↗

(On Diff #201218)

Stray patch?

[c-rhodes]

test/MC/AArch64/SVE/movprfx-diagnostics.s

114 ↗

(On Diff #201218)

This is just testing indexed + movprfxable gives a diagnostic. I understand it appears to be a rogue patch given the instruction is unrelated but the reason for adding it here was we noticed there were no tests for this instruction form in movprfx-diagnostics, initially thinking indexed + movprfxable was new to SVE2 until realising the SVE SDOT instruction is similar, so a test was added for that.

[chill]

test/MC/AArch64/SVE2/fmlalb-diagnostics.s

42 ↗

(On Diff #201218)

Missing invalid width tests for vectors (non-indexed) version, e.g.:

fmlalb z0.s, z1.d, z2.d
// CHECK: [[@LINE-1]]:{{[0-9]+}}: error: invalid element width
// CHECK-NEXT: fmlalb z0.s, z1.d, z2.d
// CHECK-NOT: [[@LINE-1]]:{{[0-9]+}}:

Likewise for fmlslb, fmlslt, and fmlalt.

[chill]

LGTM. But, please, consider the extra tests.