Reapply [InstCombine] Remove early constant fold

The reported compile-time regression has been address in
47f9109d.

Additionally, this contains a change to immediately fold zext
with constant operand, even if it's used in a trunc. I'm not sure
if this is relevant for anything, but I noticed it as a behavioral
discrepancy when investigating this issue.

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InstCombine currently performs a constant folding attempt as part
of the main InstCombine loop, before visiting the instruction.
However, each visit method will also attempt to simplify the
instruction, which will in turn constant fold it. (Additionally,
we also constant fold instructions before the main InstCombine loop
and use a constant folding IR builder, so this is doubly redundant.)

There is one place where InstCombine visit methods currently don't
call into simplification, and that's casts. To be conservative,
I've added an explicit constant folding call there (though it has
no impact on tests).

This makes for a mild compile-time improvement and in particular
mitigates the compile-time regression from enabling load
simplification in be88b581.

Differential Revision: https://reviews.llvm.org/D144369