AArch64: use RegisterOperand for NEON registers.

Previously we modelled VPR128 and VPR64 as essentially identical
register-classes containing V0-V31 (which had Q0-Q31 as "sub_alias"
sub-registers). This model is starting to cause significant problems
for code generation, particularly writing EXTRACT/INSERT_SUBREG
patterns for converting between the two.

The change here switches to classifying VPR64 & VPR128 as
RegisterOperands, which are essentially aliases for RegisterClasses
with different parsing and printing behaviour. This fits almost
exactly with their real status (VPR128 == FPR128 printed strangely,
VPR64 == FPR64 printed strangely).