[NFC][SimplifyCFG] 'Safely extract low bits' pattern will also benefit from...

[NFC][SimplifyCFG] 'Safely extract low bits' pattern will also benefit from -phi-node-folding-threshold=3

This is the naive implementation of x86 BZHI/BEXTR instruction:
it takes input and bit count, and extracts low nbits up to bit width.
I.e. unlike shift it does not have any UB when nbits >= bitwidth.
Which means we don't need a while PHI here, simple select will do.
And if it's a select, it should then be trivial to fix codegen
to select it to BEXTR/BZHI.

See https://bugs.llvm.org/show_bug.cgi?id=34704

llvm-svn: 370369