Make a transformation added in 63266 a bit less aggressive.

It was transforming (x&y)==y to (x&y)!=0 in the case where
y is variable and known to have at most one bit set (e.g. z&1).
This is not correct; the expressions are not equivalent when y==0.
I believe this patch salvages what can be salvaged, including
all the cases in bt.ll.  Dan, please review.
Fixes gcc.c-torture/execute/20040709-[12].c

llvm-svn: 64314