Fixes #169017

We already know we're looking at BITREVERSE, we can match on the source
operand.

This reverts commit 6d5f87fc4284c4c22512778afaf7f2ba9326ba7b.

Previously this failed due to treating the unknown MachineMemOperand
value as known uniform.

To make life easier for future contributors. Note that formatting
changes are due to git clang-format on the touched whitespace-error
lines.

Currently LibcallLoweringInfo is defined inside of TargetLowering,
which is owned by the subtarget. Pass in the subtarget so we can
construct LibcallLoweringInfo with the subtarget. This is a temporary
step that should be revertable in the future, after LibcallLoweringInfo
is moved out of TargetLowering.

Fixes #167710

If vector-unaligned-mem support is not enabled, we should not generate
loads/stores that are not aligned to their element size.

We already do this for non-VP vector loads/stores.

This code has been in our downstream for about a year and a half after
finding the vectorizer generating misaligned loads/stores. I don't think
that is unique to our downstream.

Doing this for masked vp.load/store requires widening the mask as well
which is harder to do.

NOTE: Because we have to scale the VL, this will introduce additional
vsetvli and the VL optimizer will not be effective at optimizing any
arithmetic that is consumed by the store.