[LoopFusion] Bails out if only the second candidate is guarded (PR48060)

If only the second candidate loop is guarded while the first one is not, fusioning
two loops might not be valid but this check is currently missing.

Fixes https://bugs.llvm.org/show_bug.cgi?id=48060

Reviewed By: sidbav

Differential Revision: https://reviews.llvm.org/D99716