[LoopUnroll] Remove remapInstruction().

Summary:
LoopUnroll can reuse the RemapInstruction() in ValueMapper, or
remapInstructionsInBlocks() in CloneFunction, depending on the needs.
There is no need to have its own version in LoopUnroll.

By calling RemapInstruction() without TypeMapper or Materializer and
with Flags (RF_NoModuleLevelChanges | RF_IgnoreMissingLocals), it does
the same as remapInstruction(). remapInstructionsInBlocks() calls
RemapInstruction() exactly as described.

Looking at the history, I cannot find any obvious reason to have its own
version.
Reviewer: dmgreen, jdoerfert, Meinersbur, kbarton, bmahjour, etiotto,
foad, aprantl
Reviewed By: jdoerfert
Subscribers: hiraditya, zzheng, llvm-commits, prithayan, anhtuyen
Tag: LLVM
Differential Revision: https://reviews.llvm.org/D73277