[llvm] [Thumb] Resolve FIXME: Transform "(and (shl x, c2), c1)" into "(shl (and x, c1>>c2), c2)" (PR #82120)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

> @RKSimon what do you think about merging this?

I don't think its necessary - we're up top 18.1.6 now and this is a perf improvement, not a critical bugfix for a recent issue. 

Better to focus on getting trunk ready for 19.X in a couple of months time.

https://github.com/llvm/llvm-project/pull/82120