[AArch64][SVE][InstCombine] Replace last{a,b} intrinsics with extracts...

when the predicate used by last{a,b} specifies a known vector length.

For example:
  aarch64_sve_lasta(VL1, D) -> extractelement(D, #1)
  aarch64_sve_lastb(VL1, D) -> extractelement(D, #0)

Co-authored-by: Paul Walker <paul.walker@arm.com>

Differential Revision: https://reviews.llvm.org/D100476