Add movabs -> mov shortening optimization. Add peephole optimization pass...

Add movabs -> mov shortening optimization.  Add peephole optimization pass that does instruction shortening.

Summary:
Shorten when a mov instruction has a 64-bit immediate that can be repesented as
a sign extended 32-bit number, use the smaller mov instruction (MOV64ri -> MOV64ri32).

Add peephole optimization pass that does instruction shortening.

(cherry picked from FBD3603099)