[LoopVectorize] Use ReplaceInstWithInst() helper where appropriate.

This is mostly an NFC, which increases code readability (instead of
saving old terminator, generating new one in front of old, and deleting
old, we just call a function). However, it would additionaly copy
the debug location from old instruction to replacement, which
would help PR23837.

llvm-svn: 241197