[X86][SSE] getFauxShuffle - don't combine shuffles with small truncated scalars (PR45604)

getFauxShuffle attempts to combine INSERT_VECTOR_ELT(TRUNCATE/EXTEND(EXTRACT_VECTOR_ELT(x))) patterns into a target shuffle chain.

PR45604 identified an issue where the scalar was truncated to a size smaller than the destination vector element and then zero extended back, which requires the upper bits to be zero'd which we don't currently do.

To avoid the bug I've added an early out in these truncation cases, a future commit should allow us to handle this by inserting the necessary SM_SentinelZero padding.