Optimize some special cases for SSE4a insertqi

Summary:
Since the upper 64 bits of the destination register are undefined when
performing this operation, we can substitute it and let the optimizer
figure out that only a copy is needed.

Also added range merging, if an instruction copies a range that can be
merged with a previous copied range.

Added test cases for both optimizations.

Reviewers: grosbach, nadav

CC: llvm-commits

Differential Revision: http://reviews.llvm.org/D3357