release/18.x: [SystemZ] Fix overflow flag for i128 USUBO (#86491)

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