[LoopFuse] Ensure loops are in loop simplified form under new PM

Loop Fusion (Function Pass) requires loops in simplified form. With
legacy-pm, loop-simplify pass is added as a dependency for loop-fusion.
But the new pass manager does not always ensure this format. This patch
tries to invoke simplifyLoop() on loops that are not in simplified form
only for new PM.

Reviewed By: aeubanks

Differential Revision: https://reviews.llvm.org/D136781