[LLVM][CodeGen][AArch64] Simplify lowering for predicate inserts. (#89072)

The original code has an invalid use of UZP1 because the result vector
type does not match its input vector types. Rather than insert extra nop
casts I figure it would be better to use CONCAT_VECTORS because that's
the operation we're performing.

NOTE: This is a step to enable more asserts in verifyTargetSDNode.