[x86] Switch to a formulation of a for loop that is much more obviously

[x86] Switch to a formulation of a for loop that is much more obviously
not corrupting the mask by mutating it more times than intended. No
functionality changed (the results were non-overlapping so the old
version "worked" but was non-obvious).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214953 91177308-0d34-0410-b5e6-96231b3b80d8