DebugInfo: Add a method to find debug info for replacements and use it in Precompute

[kripken]

Many optimizations just move code around, and debug info is preserved anyhow as we
do so. Precompute however constantly creates new constant nodes, so we were losing
a significant amount of debug info.

Just copying debug info for the replacement is not enough, as we may replace a parent
with a child, like this:

(block
  ;; debug info on the const
  (i32.const 42)
)
=>
(i32.const 42)

When replacing the block we should look for debug info anywhere inside it, if we have
anything else, that is, debug info on a child is probably better than not having any
debug info at all.

This helps save debug info after inlining in some cases, see the new testcases.

[codecov]

Codecov Report

Merging #5900 (30a3df7) into main (0598939) will increase coverage by 0.01%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##             main    #5900      +/-   ##
==========================================
+ Coverage   42.57%   42.58%   +0.01%     
==========================================
  Files         484      485       +1     
  Lines       74831    74845      +14     
  Branches    11924    11931       +7     
==========================================
+ Hits        31858    31876      +18     
+ Misses      39772    39762      -10     
- Partials     3201     3207       +6     

Files Changed	Coverage Δ
src/ir/debug.h	85.71% <ø> (+85.71%)	⬆️
src/ir/debug.cpp	84.61% <84.61%> (ø)
src/passes/Precompute.cpp	80.00% <100.00%> (+0.25%)	⬆️

... and 7 files with indirect coverage changes

[aheejin]

Is this generally helpful even when the replacement is not a child of the original? In case it is not, is inaccurate info better than no info?

[kripken]

I think it is generally helpful, but maybe we can think of an example that's not true? My thinking, at least, is that if we replace A(B, C) with some D then D does what all of A(B, C) did, so it's better to use the source file location for say C if we don't have one for A - it's at least representing the location of some of the source code that D implements.

[kripken]

This is probably still worth landing but I'll wait for other pending PRs in this area to land first, as this is the least important. We may also want a more general fix here, I'm not sure.