A miscompilation issue has been addressed with improved handling.

Fixes: https://github.com/llvm/llvm-project/issues/133928.

Alive2: https://alive2.llvm.org/ce/z/gcMNvS.

As part of the "RemoveDIs" project, BasicBlock::iterator now carries a
debug-info bit that's needed when getFirstNonPHI and similar feed into
instruction insertion positions. Call-sites where that's necessary were
updated a year ago; but to ensure some type safety however, we'd like to
have all calls to moveBefore use iterators.

This patch adds a (guaranteed dereferenceable) iterator-taking
moveBefore, and changes a bunch of call-sites where it's obviously safe
to change to use it by just calling getIterator() on an instruction
pointer. A follow-up patch will contain less-obviously-safe changes.

We'll eventually deprecate and remove the instruction-pointer
insertBefore, but not before adding concise documentation of what
considerations are needed (very few).

This used to be necessary to fetch the DataLayout, but isn't anymore.

Similar to https://github.com/llvm/llvm-project/pull/96902, this adds
`getDataLayout()` helpers to Function and GlobalValue, replacing the
current `getParent()->getDataLayout()` pattern.

We currently check if the source and promoted types are not equal before
generating truncate instructions. This does not work for RV64 where the
promoted type is i64 and this lead to a crash due to the generation of
truncate instructions from i32 to i64.

Fixes #86400

With special case with Add constant is 0.

Original message:
We can support these by changing the sext promotion to -zext(-C) and
replacing a sgt check with ugt. Reframing the logic in terms of how the
unsigned range are affected. More comments in the patch.

The new cases check isLegalAddImmediate to avoid some
regressions in lit tests.

This reverts commit 0813b90ff5d195d8a40c280f6b745f1cc43e087a.

Fixes miscompile reported in #84718.

We can support these by changing the sext promotion to -zext(-C) and
replacing a sgt check with ugt. Reframing the logic in terms of how the
unsigned range are affected. More comments in the patch.

The new cases check isLegalAddImmediate to avoid some
regressions in lit tests.