[ARM] Constant Materialize: imms with specific value can be encoded into mov.w

Summary: Thumb2 supports encoding immediates with specific patterns into mov.w by splatting the low 8 bits into other bytes.

I'm resubmitting this patch. The test case in the original commit
r277610 does not specify triple, so builds with differnt default triple
will have different output.

This patch fixed trile as thumb-darwin-apple.

Reviewers: john.brawn, jmolloy, bruno

Subscribers: jmolloy, aemerson, rengolin, samparker, llvm-commits

Differential Revision: https://reviews.llvm.org/D23090

llvm-svn: 277865