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

LoopInfo doesn't give all loops in a loop nest, it gives top level loops
only. While isLoopSimplifyForm() only checkes for the outter most loop of a
loop nest. As a result, inner loops that are not in simplied form can
not be simplified with the original code.

Reviewed By: aeubanks

Differential Revision: https://reviews.llvm.org/D137672