assert_instr: add support for stricter tests

[RalfJung]

Motivated by #2208, this extends assert_instr with support for stricter tests:

• The first commit adds clauses of the form not(instr); the macro will then ensure that the given instruction does not appear in the disassembly.
• The second commit adds clauses of the form limit(n); the macro will ensure that overall there are no more than n instructions in the disassembly.

I verified that the problematic pack tests indeed fail before the revert of #2033. I am not sure if the not part is even worth it, which is why the 2nd commit removes its uses -- one has to have very specific instructions in mind to exclude them, a general limit on how many instructions total there are seems better? So unless you think I should keep it I plan to remove the first commit from this again.

r? @folkertdev

[RalfJung]

I am also confused by ci/run.sh... it seems to never even run the AVX(2) tests with assert_instr turned on...?

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[RalfJung]

Ah... it's 5 instructions somewhere. That's annoying.

disassembly for stdarch_test_shim__mm256_packs_epi16_vpacksswb: 
	 0: pushq %rbp
	 1: movq %rsp, %rbp
	 2: vpacksswb %ymm1, %ymm0, %ymm0
	 3: popq %rbp
	 4: retq

Maybe it should ignore some of these as they have nothing to do with the actual intrinsic?