[X86] Teach X86FixupBWInsts to promote MOV8rr/MOV16rr to MOV32rr.

This re-applies r268760, reverted in r268794.
Fixes http://llvm.org/PR27670

The original imp-defs assertion was way overzealous: forward all
implicit operands, except imp-defs of the new super-reg def (r268787
for GR64, but also possible for GR16->GR32), or imp-uses of the new
super-reg use.
While there, mark the source use as Undef, and add an imp-use of the
old source reg: that should cover any case of dead super-regs.

At the stage the pass runs, flags are unlikely to matter anyway;
still, let's be as correct as possible.

Also add MIR tests for the various interesting cases.

Original commit message:
Codesize is less (16) or equal (8), and we avoid partial
dependencies.

Differential Revision: http://reviews.llvm.org/D19999

llvm-svn: 268831