[AggressiveInstCombine] Combine consecutive loads which are being merged to form a wider load.

The patch simplifies some of the patterns as below

1. (ZExt(L1) << shift1) | (ZExt(L2) << shift2) -> ZExt(L3) << shift1
2. (ZExt(L1) << shift1) | ZExt(L2) -> ZExt(L3)

The pattern is indicative of the fact that the loads are being merged to a wider load and the only use of this pattern is with a wider load. In this case for a non-atomic/non-volatile loads reduce the pattern to a combined load which would improve the cost of inlining, unrolling, vectorization etc.

Fix the error reported on reverse load merge.

Differential Revision: https://reviews.llvm.org/D127392