[LoopVec] Support non-instructions as argument to uniform mem ops

The initial step of the uniform-after-vectorization (lane-0 demanded only) analysis was very awkwardly written. It would revisit use list of each pointer operand of a widened load/store. As a result, it was in the worst case O(N^2) where N was the number of instructions in a loop, and had restricted operand Value types to reduce the size of use lists.

This patch replaces the original algorithm with one which is at most O(2N) in the number of instructions in the loop. (The key observation is that each use of a potentially interesting pointer is visited at most twice, once on first scan, once in the use list of *it's* operand. Only instructions within the loop have their uses scanned.)

In the process, we remove a restriction which required the operand of the uniform mem op to itself be an instruction.  This allows detection of uniform mem ops involving global addresses.

Differential Revision: https://reviews.llvm.org/D92056

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