Revert reserved immediate word shifts with imm[5]!=0

[mininny]

Description

As per specification: "SLLIW, SRLIW, and SRAIW encodings with imm[5] != 0 are reserved".

However, implementations (sol/fast/slow) accept it.

This PR reverts the reserved SLLIW/SRLIW/SRAIW instructions with imm[5] != 0

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 25.00000% with 12 lines in your changes missing coverage. Please review.

Project coverage is 61.64%. Comparing base (45c520d) to head (f4452bd).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
rvgo/fast/vm.go	25.00%	4 Missing and 2 partials ⚠️
rvgo/slow/vm.go	25.00%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #105      +/-   ##
==========================================
- Coverage   61.78%   61.64%   -0.15%     
==========================================
  Files          27       27              
  Lines        4132     4148      +16     
==========================================
+ Hits         2553     2557       +4     
- Misses       1436     1444       +8     
- Partials      143      147       +4     

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[refcell]

Nice!