[GlobalISel] Support for inlining memcpy, memset and memmove calls.

This introduces a new family of combiner helper routines that re-use the
target specific cost model from SelectionDAG, and generate inline implementations
of the memcpy family of intrinsics.

The combines are only enabled at optimization levels higher than -O0, and give
very substantial performance improvements.

Differential Revision: https://reviews.llvm.org/D65167

llvm-svn: 366951