Recent upstream trends have moved away from explicitly using `-verify-machineinstrs`, as it's already covered by the expensive checks. This PR removes almost all `-verify-machineinstrs` from tests in `llvm/test/CodeGen/AMDGPU/*.ll`, leaving only those tests where its removal currently causes failures.

For the majority of cases, this is a neutral or positive change.
There are even testcases that greatly benefit from it, but some regressions are possible.
There is #140040 for GlobalISel that'd need to be fixed but it's only a one instruction regression and I think it can be fixed later.

Solves #64591

Many tests use store to undef as a placeholder use, so just replace
all of these with poison.

This is the bulk update with perl, with cases which require additional
update left for later.

This reverts commit ca33649abe5fad93c57afef54e43ed9b3249cd86.

This reverts commit e215a1e27d84adad2635a52393621eb4fa439dc9 as it broke both
hip and openmp buildbots.

Make some minor tweaks to AMDGPU tests to ensure they still work as
intended after https://github.com/llvm/llvm-project/pull/97762. These
tests can be radically simplified after bitcast aware fpclass deduction.

This reverts commit adaff46d087799072438dd744b038e6fd50a2d78.

Drop the -O3 checks from default-attributes.hip. I don't know why they
are different on some bots but reverting this is far too disruptive.