Revert "[LICM] Support hosting of dynamic allocas out of loops"

This reverts commit 8d22100f.

There was a functional regression reported (https://bugs.llvm.org/show_bug.cgi?id=44996).  I'm not actually sure the patch is wrong, but I don't have time to investigate currently, and this line of work isn't something I'm likely to get back to quickly.

(cherry picked from commit 14845b2c)