[X86] Combining CMOVs with [ANY,SIGN,ZERO]_EXTEND for cases where CMOV has constant arguments

Combine CMOV[i16]<-[SIGN,ZERO,ANY]_EXTEND to [i32,i64] into CMOV[i32,i64].
One example of where it is useful is:

before (20 bytes)
    <foo>:
    test $0x1,%dil
    mov $0x307e,%ax
    mov $0xffff,%cx
    cmovne %ax,%cx
    movzwl %cx,%eax
    retq

after (18 bytes)
    <foo>:
    test $0x1,%dil
    mov $0x307e,%ecx
    mov $0xffff,%eax
    cmovne %ecx,%eax
    retq

Reviewers: craig.topper, aaboud, spatel, RKSimon, zvi

Reviewed By: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D36711

llvm-svn: 313982