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[GVNHoist] safeToHoistLdSt allows illegal hoisting

Description

[GVNHoist] safeToHoistLdSt allows illegal hoisting

Bug fix for PR36787. When reasoning if it's safe to hoist a load we
want to make sure that the defining memory access dominates the new
insertion point of the hoisted instruction. safeToHoistLdSt calls
firstInBB(InsertionPoint,DefiningAccess) which returns false if
InsertionPoint == DefiningAccess, and therefore it falsely thinks
it's safe to hoist.

Differential Revision: https://reviews.llvm.org/D49555