[AArch64]  Lower multiplication by a constant int to shl+add+shl

Lower a = b * C where C = (2^n + 1) * 2^m to

add     w0, w0, w0, lsl n
lsl     w0, w0, m

Differential Revision: https://reviews.llvm.org/D229245

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@287019 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed