[X86][MMX] Fix folding of shift value loads to cover whole 64-bits

rL230225 made the assumption that only the lower 32-bits of an MMX register load is used as a shift value, when in fact the whole 64-bits are reloaded and treated as a i64 to determine the shift value.

This patch reverts rL230225 to ensure that the whole 64-bits of memory are folded and ensures that the upper 32-bit are zero'd for cases where the shift value has come from a scalar source.

Found during fuzz testing.

Differential Revision: https://reviews.llvm.org/D30833

llvm-svn: 297667