X86: Shrink certain forms of movsx.

In particular:
movsbw %al, %ax   --> cbtw
movswl %ax, %eax  --> cwtl
movslq %eax, %rax --> cltq

According to Intel's manual those have the same performance characteristics but
come with a smaller encoding.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186174 91177308-0d34-0410-b5e6-96231b3b80d8