[PowerPC] Combine ADD to ADDZE

On the ppc64le platform, if ir has the following form,

define i64 @addze1(i64 %x, i64 %z) local_unnamed_addr #0 {
entry:
  %cmp = icmp ne i64 %z, CONSTANT      (-32767 <= CONSTANT <= 32768)
  %conv1 = zext i1 %cmp to i64
  %add = add nsw i64 %conv1, %x
  ret i64 %add
}
we can optimize it to the form below.

                                when C == 0
                            --> addze X, (addic Z, -1))
                           /
add X, (zext(setne Z, C))--
                           \    when -32768 <= -C <= 32767 && C != 0
                            --> addze X, (addic (addi Z, -C), -1)

Patch By: HLJ2009 (Li Jia He)
Differential Revision: https://reviews.llvm.org/D51403
Reviewed By: Nemanjai 

llvm-svn: 341634