use valid bits to avoid unnecessary machine trace metric recomputations

use valid bits to avoid unnecessary machine trace metric recomputations

Although this does cut the number of traces recomputed by ~10% for the
test case mentioned in http://reviews.llvm.org/D10460, it doesn't
make a dent in the overall performance. That example needs to be more
selective when invalidating traces.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241393 91177308-0d34-0410-b5e6-96231b3b80d8