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
