[ARM] MVE Vector Shifts

This adds basic lowering for MVE shifts. There are many shifts in MVE, but the
instructions handled here are:
 VSHL (imm)
 VSHRu (imm)
 VSHRs (imm)
 VSHL (vector)
 VSHL (register)

MVE, like NEON before it, doesn't have shift right by a vector (or register).
We instead have to negate the amount and shift in the opposite direction. This
means we have to convert any SHR's into a form of SHL (that is still signed or
unsigned) with a negated condition and selecting from there. MVE still does
have shifting by an immediate for SHL, ASR and LSR.

This adds lowering for these and for register forms, which work well for shift
lefts but may require an extra fold of neg(vdup(x)) -> vdup(neg(x)) to potentially
work optimally for right shifts.

Differential Revision: https://reviews.llvm.org/D64212

llvm-svn: 366056