This patch removes unnecessary zero copies in BBs that are targets of b.eq/b.ne
and we know the result of the compare instruction is zero. For example,
BB#0:
subs w0, w1, w2
str w0, [x1]
b.ne .LBB0_2
BB#1:
mov w0, wzr ; <-- redundant
str w0, [x2]
.LBB0_2
Differential Revision: https://reviews.llvm.org/D35075
llvm-svn: 308849
