Fix overflow flag for i128 USUBO

We use the VSCBIQ/VSBIQ/VSBCBIQ family of instructions to implement
USUBO/USUBO_CARRY for the i128 data type.  However, these instructions
use an inverted sense of the borrow indication flag (a value of 1
indicates *no* borrow, while a value of 0 indicated borrow).  This
does not match the semantics of the boolean "overflow" flag of the
USUBO/USUBO_CARRY ISD nodes.

Fix this by generating code to explicitly invert the flag.  These
cancel out of the result of USUBO feeds into an USUBO_CARRY.

To avoid unnecessary zero-extend operations, also improve the
DAGCombine handling of ZERO_EXTEND to optimize (zext (xor (trunc)))
sequences where appropriate.

Fixes: https://github.com/llvm/llvm-project/issues/83268

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