[PowerPC] multiply-with-overflow might use the CTR register

Check the legality of ISD::[US]MULO to see whether
Intrinsic::[us]mul_with_overflow will legalize into a function call (and, thus,
will use the CTR register).  Fixes PR32485.

Patch by Tim Neumann!

Differential Revision: https://reviews.llvm.org/D31790

llvm-svn: 299910