Limit domain conversion to cases where it won't break dep chains.

Limit domain conversion to cases where it won't break dep chains.

NEON domain conversion was too heavy-handed with its widened
registers, which could have stripped existing instructions of their
dependency, leaving them vulnerable to scheduling errors.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@163070 91177308-0d34-0410-b5e6-96231b3b80d8