[LV] Use Legal::isUniform to detect uniform pointers.

Update collectLoopUniforms to identify uniform pointers using
Legal::isUniform. This is more powerful and  brings pointer
classification here in sync with setCostBasedWideningDecision
which uses isUniformMemOp. The existing mis-match in reasoning
can causes crashes due to D134460, which is fixed by this patch.

Fixes https://github.com/llvm/llvm-project/issues/60831.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D150991