[SDAG] bail out of mergeTruncStores() if there's any other use in the chain

This fixes the miscompile in issue #58883.

The test demonstrates that we gave up on store merging in that example.

This change should be strictly safe (just adds another clause
to avoid the transform), and it does not prohibit any existing
valid optimizations based on regression tests. I want to believe
that it's also a sufficient fix (possibly overkill), but I'm not
sure how to prove that.

Differential Revision: https://reviews.llvm.org/D137791