Split from #131312

There are a number of backends (specifically AArch64, AMDGPU, Mips, and
RISCV) which contain a “TODO: make CombinerHelper methods const”
comment. This PR does just that and makes all of the CombinerHelper
methods const, removes the TODO comments and makes the associated
instances const. This change makes some sense because the CombinerHelper
class simply modifies the state of _other_ objects to which it holds
pointers or references.

Note that AMDGPU contains an identical comment for an instance of
AMDGPUCombinerHelper (a subclass of CombinerHelper). I deliberately
haven’t modified the methods of that class in order to limit the scope
of the change. I’m happy to do so either now or as a follow-up.

This patch fixes:

  llvm/lib/CodeGen/GlobalISel/CombinerHelperCasts.cpp:167:21: error:
  unused variable 'DL' [-Werror,-Wunused-variable]

  llvm/lib/CodeGen/GlobalISel/LoadStoreOpt.cpp:320:15: error: unused
  variable 'DL' [-Werror,-Wunused-variable]

trunc (binop X, C) --> binop (trunc X, trunc C)  --> binop (trunc X, C`)

Try to narrow the width of math or bitwise logic instructions by pulling
a truncate ahead of binary operators.

Vx and Nx cores consider 32-bit and 64-bit basic arithmetic equal in
costs.

Credits: https://github.com/llvm/llvm-project/pull/100563

Credits:
https://reviews.llvm.org/D86516

combine-ext.mir

Notable semantic changes:
InstCombine does not mix zero and sign extend,
  see CastInst::isEliminableCastPair.
New version has legality checks.
Folds sext/zext of anyext -> sext/zext
Support for nneg

Future work:
nneg zext of sext -> sext
sext of nneg zext -> sext

Credits:
https://github.com/llvm/llvm-project/pull/90964
https://reviews.llvm.org/D87050

combine-trunc.mir

Functional changes intended.