[APInt] Merge the multiword code from lshrInPlace and tcShiftRight into a single implementation

This merges the two different multiword shift right implementations into a single version located in tcShiftRight. lshrInPlace now calls tcShiftRight for the multiword case.

I retained the memmove fast path from lshrInPlace and used a memset for the zeroing. The for loop is basically tcShiftRight's implementation with the zeroing and the intra-shift of 0 removed.

Differential Revision: https://reviews.llvm.org/D32114

llvm-svn: 300503