[SimplifyCFG] Do not hoist/sink convergent function calls

Currently SimplifyCFG hoists/sink common instructions in then/else basic blocks
when certain options are enabled, which is the case for default clang optimization
pipelines for -O3. It tries to hoist/sink convergent function calls in divergent
control flow, which causes incorrect ISA generated for GPU, e.g.
https://github.com/ROCm-Developer-Tools/HIP/issues/3172

This patch fixes that by conservatively disable hoisting/sinking common
convergent function calls in then/else blocks.

Reviewed by: Artem Belevich

Differential Revision: https://reviews.llvm.org/D144756