[ConstantFold] Get rid of special cases for sizeof etc.

Target-dependent constant folding will fold these down to simple
constants (or at least, expressions that don't involve a GEP).  We don't
need heroics to try to optimize the form of the expression before that
happens.

Fixes https://bugs.llvm.org/show_bug.cgi?id=51232 .

Differential Revision: https://reviews.llvm.org/D107116