feat[venom]: mark loads as non-volatile

[charles-cooper]

What I did

How I did it

How to verify it

Commit message

this commit marks load instructions (`mload`, `sload`,
etc) as non-volatile, allowing them to be removed in the
`remove_unused_variables` pass.

in some cases, we rely on the `msize` modifying behavior of
`iload`. since this is a special case, we add an `itouch` instruction
which is specifically volatile and emitting it from the frontend,
allowing the other usages of `iload` to be removed.

this also adds a reachability analysis. it was used in an older version
of this PR but is no longer used, but it might be generally useful
(e.g. in loop detection) so we leave it in.

Description for the changelog

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[codecov]

Codecov Report

Attention: Patch coverage is 66.03774% with 18 lines in your changes missing coverage. Please review.

Project coverage is 91.89%. Comparing base (f444c8f) to head (8d93190).

Files with missing lines	Patch %	Lines
vyper/venom/analysis/reachable.py	40.00%	15 Missing ⚠️
vyper/venom/venom_to_assembly.py	71.42%	1 Missing and 1 partial ⚠️
vyper/utils.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4388      +/-   ##
==========================================
- Coverage   91.98%   91.89%   -0.09%     
==========================================
  Files         119      120       +1     
  Lines       16686    16732      +46     
  Branches     2808     2814       +6     
==========================================
+ Hits        15348    15376      +28     
- Misses        919      936      +17     
- Partials      419      420       +1     

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[charles-cooper]

@HodanPlodky pointed out offline -- there could be an msize instruction that this optimization interferes with

[HodanPlodky]

Should not this store last msize in basic block instead of first msize. This could be the problem in case which is showed in PR charles-cooper#53

[charles-cooper]

i investigated refactoring (or at least trimming down) VOLATILE_INSTRUCTIONS and replacing it with essentially checking if the instruction is a terminator instruction or has write effects. however, it was not that clean since we still need to special-case MSIZE.

another approach which simplifies the code here would be to have a special volatile instruction like mtouch which has an msize write effect, and then have the frontend issue it specifically when it needs to fence msize. i think this approach is OK - i think the frontend only depends on the "memory reservation" capability of mload (actually iload) in one place.