Fix ExpandShiftWithUnknownAmountBit, which was completely bogus.

Pointed out by Javier Martinez (who also provided a patch).  Since
this logic is not used on (for example) x86, I guess nobody noticed.
Tested by generating SHL, SRL, SRA on various choices of i64 for all
possible shift amounts, and comparing with gcc.  Since I did this on
x86-32, I had to force the use of ExpandShiftWithUnknownAmountBit.
What I'm saying here is that I don't have a testcase I can add to the
repository.

llvm-svn: 90482