fix: fixes issue #228

[baszalmstra]

Fixes a few issues (presented in #228):

• In IR: When a block had a tailing expression with a never type the block would return an empty struct.
• If the then block of an if/else expression had a never type, it would return an empty struct instead of the result of the if statement.
• If the then block and the else block have a never type, the if would return an empty struct.

[codecov]

Codecov Report

Merging #229 into master will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #229      +/-   ##
==========================================
+ Coverage   81.06%   81.13%   +0.06%     
==========================================
  Files         182      182              
  Lines       11933    11938       +5     
==========================================
+ Hits         9674     9686      +12     
+ Misses       2259     2252       -7     

Impacted Files	Coverage Δ
crates/mun_codegen/src/ir/body.rs	84.96% <100.00%> (-0.09%)	⬇️
crates/mun_codegen/src/test.rs	98.23% <100.00%> (+0.03%)	⬆️
crates/mun_target/src/abi/mod.rs	56.73% <0.00%> (+3.84%)	⬆️
crates/mun_hir/src/ty/primitives.rs	68.18% <0.00%> (+4.54%)	⬆️
crates/mun_hir/src/ty/resolve.rs	68.75% <0.00%> (+6.25%)	⬆️

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