fix(gnovm): handle non call expression valuedecl values

[deelawn]

Closes #2636.

With a value declaration like var a, b = ..., we currently only support a call expression on the RHS. This PR supports type assertion and index expressions as well.

Contributors' checklist...

• Added new tests, or not needed, or not feasible
• Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• Updated the official documentation or not needed
• No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• Added references to related issues and PRs
• Provided any useful hints for running manual tests
• Added new benchmarks to generated graphs, if any. More info here.

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 60.16%. Comparing base (5f28803) to head (dd9f730).
Report is 19 commits behind head on master.

Files	Patch %	Lines
gnovm/pkg/gnolang/preprocess.go	91.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2647      +/-   ##
==========================================
+ Coverage   60.14%   60.16%   +0.02%     
==========================================
  Files         560      561       +1     
  Lines       74738    75507     +769     
==========================================
+ Hits        44950    45431     +481     
- Misses      26400    26663     +263     
- Partials     3388     3413      +25     

Flag	Coverage Δ
contribs/gnodev	61.40% <ø> (+0.81%)	⬆️
contribs/gnofaucet	14.46% <ø> (ø)
gno.land	64.75% <ø> (+0.59%)	⬆️
gnovm	64.16% <91.66%> (-0.12%)	⬇️
misc/genstd	80.54% <ø> (ø)
misc/logos	19.88% <ø> (ø)
tm2	62.09% <ø> (+0.07%)	⬆️

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[thehowl]

ok; i still think the problem is that we should not have different code for AssignStmt.DECL and DeclStmt. but it's fine to fix the issue directly.

[piux2]

LGTM, Thank you!