[GVN] Patch for invalid GVN replacement

If PRE is performed as part of the main GVN pass (to PRE GEP
operands before processing loads), and it is performed across a
backedge, we will end up adding the new instruction to the leader
table of a block that has not yet been processed. When it will be
processed, GVN will incorrectly assume that the value is already
available, even though it is only available at the end of the
block.

Avoid this by not performing PRE across backedges.

Fixes https://github.com/llvm/llvm-project/issues/58418.

Differential Revision: https://reviews.llvm.org/D136095