[X86] Convert esp-relative movs of function arguments into pushes, step 1
This handles the simplest case for mov -> push conversion:
- x86-32 calling convention, everything is passed through the stack.
- There is no reserved call frame.
- Only registers or immediates are pushed, no attempt to combine a mem-reg-mem sequence into a single PUSHmm.
Differential Revision: http://reviews.llvm.org/D6503