[X86] Use SmallVector::assign instead of resize to ensure we really start with a...

[X86] Use SmallVector::assign instead of resize to ensure we really start with a vector of all -1s. Otherwise we're trusting the caller to pass the right thing.

This should be no functional change with current code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@273113 91177308-0d34-0410-b5e6-96231b3b80d8