Disable SimplifyToExp in MatchBranchSimplification

[dianqk]

Due to the miscompilation mentioned in #124150, We need to disable MatchBranchSimplification temporarily.

To fully resolve this issue, my plan is:

1. Disable SimplifyToExp in MatchBranchSimplification (this PR).
2. Remove all potentially unclear transforms in Don't perform unsigned comparisons for signed integers  #124122.
3. Gradually add back the removed transforms (possibly multiple PRs).

r? @Nilstrieb or @oli-obk