[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.

Reviewers: john.brawn, jmolloy

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

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

llvm-svn: 277610