[InstCombine] Remove the undef-related workaround code in visitSelectInst

This patch removes an old hack in visitSelectInst that was written to avoid miscompilation bugs in loop unswitch.
(Added via https://reviews.llvm.org/D35811)

The legacy loop unswitch pass will be removed after D124376, and the new simple loop unswitch pass correctly uses freeze to avoid introducing UB after D124252.

Since the hack is not necessary anymore, this patch removes it.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D124426