Turn X-(X-Y) into Y. According to my auto-simplifier this is the most common

Turn X-(X-Y) into Y.  According to my auto-simplifier this is the most common
simplification present in fully optimized code (I think instcombine fails to
transform some of these when "X-Y" has more than one use).  Fires here and
there all over the test-suite, for example it eliminates 8 subtractions in
the final IR for 445.gobmk, 2 subs in 447.dealII, 2 in paq8p etc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@123442 91177308-0d34-0410-b5e6-96231b3b80d8