[CGP] Limit converting phi types to simple loads and stores

Instcombine limits converting phi types to simple loads and stores. This
does the same in codegenprepare, not processing phis that are not
simple.

Note that volatile loads/store ISel will happily convert between float
and int. Atomics are more likely to always be integer. This just keeps
things simple and doesn't process either.

Differential Revision: https://reviews.llvm.org/D83770