Enabled mul folding optimization that was previously disabled
by being incorrect.
To preserve correctness, mul's operand that is not compared
with zero in select's condition is now frozen.

Related bug: https://bugs.llvm.org/show_bug.cgi?id=51286

Correctness:
https://alive2.llvm.org/ce/z/bHef7J
https://alive2.llvm.org/ce/z/QcR7sf
https://alive2.llvm.org/ce/z/vvBLzt
https://alive2.llvm.org/ce/z/jGDXgq
https://alive2.llvm.org/ce/z/3Pe8Z4
https://alive2.llvm.org/ce/z/LGga8M
https://alive2.llvm.org/ce/z/CTG5fs

Differential Revision: https://reviews.llvm.org/D108408