[SimplifyCFG] Avoid shifting by a too large exponent.

TI->getBitWidth can be > 64 and in those cases the shift will be UB due
to the exponent being too large.

To fix this, cap the shift at 63. I think this should work out fine,
because TableSize is itself a 64 bit type and the maximum table size
must fit in the type. Also, if we would underestimate the size here, at
most we get an extra ZExt.

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D124608