[AArch64][SVE] Fold constant multiply of element count

Summary:
E.g.

  %0 = tail call i64 @llvm.aarch64.sve.cntw(i32 31)
  %mul = mul i64 %0, <const>

Should emit:

  cntw    x0, all, mul #<const>

For <const> in the range 1-16.

Patch by Kerry McLaughlin

Reviewers: sdesmalen, huntergr, dancgr, rengolin, efriedma

Reviewed By: sdesmalen

Subscribers: tschuett, kristof.beyls, hiraditya, rkruppe, psnobl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71014