[VectorLegalizer] Fix uint64_t typo in ExpandUINT_TO_FLOAT (PR36391)

ExpandUINT_TO_FLOAT can accept vXi32 or vXi64 inputs, so we need to use a uint64_t shift to generate the 2^(BW/2) constant.

No test case unfortunately as no upstream target uses this, but its affecting a downstream target.

llvm-svn: 325578