[FastISel][X86] Emit more efficient instructions for integer constant materialization.

This mostly affects the i64 value type, which always resulted in an 15byte
mobavsq instruction to materialize any constant. The custom code checks the
value of the immediate and tries to use a different and smaller mov
instruction when possible.

This fixes <rdar://problem/17420988>.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215593 91177308-0d34-0410-b5e6-96231b3b80d8