[SelectionDAG] Do not crash on large integers in CheckInteger (#75787)

The CheckInteger routine called from TableGen-generated selection logic
uses getSExtValue - which will abort if the underlying APInt does not
fit into an int64_t.

This case is now triggered by the SystemZ back-end since i128 is a legal
type on certain machines. While we do not have any regular instructions
that take 128-bit immediates (like most other platforms), there are
patterns in the .td files that recognize an i128 "xor ..., -1" as a
"not".

These patterns cause code to be generated that calls the CheckInteger
routine on some i128-valued integer, which may trigger the assert.

Fix by using trySExtValue instead.

Fixes https://github.com/llvm/llvm-project/issues/75710