Seeing how we can't generate any debug intrinsics any more: delete a
variety of codepaths where they're handled. For the most part these are
plain deletions, in others I've tweaked comments to remain coherent, or
added a type to (what was) type-generic-lambdas.

This isn't all the DbgInfoIntrinsic call sites but it's most of the
simple scenarios.

Co-authored-by: Nikita Popov <github@npopov.com>

If the input has known zero bits, InstCombine may have simplied one
of the expected And masks. Teach AggressiveInstCombine to use
MaskedValueIsZero to recover these missing bits.
    
Fixes #142042.

Having a finite Depth (or recursion limit) for computeKnownBits is very
limiting, but is currently a load-bearing necessity, as all KnownBits
are recomputed on each call and there is no caching. As a prerequisite
for an effort to remove the recursion limit altogether, either using a
clever caching technique, or writing a easily-invalidable KnownBits
analysis, make the Depth argument in APIs in ValueTracking uniformly the
last argument with a default value. This would aid in removing the
argument when the time comes, as many callers that currently pass 0
explicitly are now updated to omit the argument altogether.

try_emplace can default-construct values, so we do not need to do so
on our own.  Plus, try_emplace(Key) is much shorter than
insert(std::make_pair(Key, Value()).

If we have `icmp eq or(a, shl(b)), 0` then the shift can be removed so
long as it is nuw or nsw. It is still comparing that some bits are
non-zero.
https://alive2.llvm.org/ce/z/nhrBVX.

This is also true of ne, and true for longer or chains.

When AggressiveInstCombine replaces a memchr with a switch instruction,
it currently drops the DILocation for that memchr. This patch changes
this, propagating the memchr DILocation to all the generated
instructions that replace it.

Found using https://github.com/llvm/llvm-project/pull/107279.

Proof: https://alive2.llvm.org/ce/z/r7M-Sf
Closes: #128134

Up to now, the only way to set specified FMF flags in IRBuilder is to
use `FastMathFlagGuard`. It makes the code ugly and hard to maintain.

This patch introduces a helper class `FMFSource` to replace the original
parameter `Instruction *FMFSource` in IRBuilder. To maximize the
compatibility, it accepts an instruction or a specified FMF.
This patch also removes the use of `FastMathFlagGuard` in some simple
cases.

Compile-time impact:
https://llvm-compile-time-tracker.com/compare.php?from=f87a9db8322643ccbc324e317a75b55903129b55&to=9397e712f6010be15ccf62f12740e9b4a67de2f4&stat=instructions%3Au