Allow one layer of unbalanced conditionals to be inlined

[limepoutine]

This PR enables functions with one layer of unbalanced if statements to be inlined. I want to see if there is performance benefit.

// Before inlining
if (cond)
    return exp1;
...
return exp2;

// After inlining
if (cond)
    return exp1;
else
{
    ...
    return exp2;
}

[github-actions]

DMD perf check

Metric	Base	PR	Δ
hello binary size (stripped)	0.72 MB	0.72 MB	-0.54%

All measurements

Metric	Base	PR	Δ
compile hello.d (instr)	214.9 M	214.9 M	+0.001%
compile hello.d -O -release (instr)	233.2 M	233.2 M	+0.001%
compile Phobos (instr)	5,123.6 M	5,123.6 M	0.000%
compile Phobos codegen (instr)	1,472.7 M	1,472.7 M	-0.001%
compile vibe.d (instr)	15,130.4 M	15,130.5 M	+0.001%
dmd binary size (stripped)	6.86 MB	6.86 MB	0.00%
hello binary size (stripped)	0.72 MB	0.72 MB	-0.54%
peak RSS (compile hello.d)	43.41 MB	43.43 MB	+0.05%
peak RSS (compile Phobos)	618.1 MB	617.6 MB	-0.07%
peak RSS (compile vibe.d)	1917 MB	1918 MB	+0.01%
compile dmd itself (wall)	12.2 s	12.2 s	-0.66%
compile hello.d (wall)	63.0 ms	63.0 ms	+0.04%
compile Phobos (wall)	1,546 ms	1,540 ms	-0.40%

_{3ce32e4 vs merge-base b41cb91 · about these metrics}

[limepoutine]

Tiny performance benefits. It might help more to remove other inlining blockers.