[AggressiveInstCombine] fixed PR36134

[aaboud]

Summary

Make TruncCombine class ignore unreachable basic blocks.

This should solve PR36134.

[uabelho]

I don't know this code but the fix makes sense to me and it makes the crash I saw go away.

[spatel]

LGTM.

Nit: when proposing patches and writing the commit message, make the title be more descriptive than 'Fixed Bug ###'. Ie, the title should be something like the summary text shown here:
"Make TruncCombine class ignore unreachable basic blocks"

...and then add some motivation in the summary:
"because dead code may contain non-standard IR that causes infinite looping or crashes in underlying analysis. See PR### for more details."

That lets people who are scanning the mailing list or doing bisection archaeology know quickly what this commit is doing and why.