[InstCombine] fold mask-with-signbit-splat to icmp+select

~(iN X s>> (N-1)) & Y --> (X s< 0) ? 0 : Y

https://alive2.llvm.org/ce/z/JKlQ9x

This is similar to D111410 / 727e642e ,
but it includes a 'not' of the signbit and so it
saves an instruction in the basic pattern.

DAGCombiner or target-specific folds can expand
this back into bit-hacks.

The diffs in the logical-select tests are not true
regressions - running early-cse and another round
of instcombine is expected in a normal opt pipeline,
and that reduces back to a minimal form as shown
in the duplicated PhaseOrdering test.

I have no understanding of the SystemZ diffs, so
I made the minimal edits suggested by FileCheck to
make that test pass again. That whole test file is
wrong though. It is running the entire optimizer (-O2)
to check IR, and then topping that by even running
codegen and checking asm. It needs to be split up.

Fixes #52631