[llvm][IfConversion] update successor list when merging INLINEASM_BR

If this successor list is not correct, then branch-folding may
incorrectly think that the indirect target is dead and remove it. This
results in a dangling reference to the removed block as an operand to
the INLINEASM_BR, which later will get AsmPrinted into code that doesn't
assemble.

This was made more obvious by, but is not a regression of
https://reviews.llvm.org/D130316.

Fixes: https://github.com/llvm/llvm-project/issues/60346

Reviewed By: efriedma, void

Differential Revision: https://reviews.llvm.org/D142924