[NFC][InstCombine] More test for "sign bit test via shifts" pattern (PR43595)

While that pattern is indirectly handled via
reassociateShiftAmtsOfTwoSameDirectionShifts(),
that incursme one-use restriction on truncation,
which is pointless since we know that we'll produce a single instruction.

Additionally, *if* we are only looking for sign bit,
we don't need shifts to be identical,
which isn't the case in general,
and is the blocker for me in bug in question:

https://bugs.llvm.org/show_bug.cgi?id=43595

llvm-svn: 374726