Unconditionally scan the callee before inline cost computation

[limepoutine]

This allows the test case in #23766 to be fully inlined when compiled with -inline, but does not fix the root cause that function literals in initializers nested within template mixins do not have a separate symbol table entry (so scans miss them).

Generally this should allow a handful more of functions to be inlined. Let's check out the performance impact.

BTW, this is very hard to test due to how mixins work.

[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)	5,121.6 M	5,121.6 M	+0.001%
compile Phobos codegen (instr)	1,471.9 M	1,471.9 M	0.000%
compile vibe.d (instr)	15,127.8 M	15,127.9 M	0.000%
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)	43.39 MB	43.42 MB	+0.05%
peak RSS (compile Phobos)	618.3 MB	620.5 MB	+0.36%
peak RSS (compile vibe.d)	1918 MB	1917 MB	-0.07%
compile dmd itself (wall)	12.2 s	12.4 s	+1.54%
compile hello.d (wall)	68.6 ms	67.4 ms	-1.73%
compile Phobos (wall)	1,590 ms	1,539 ms	-3.21%

_{0b231e9 vs merge-base 4264da7 · about these metrics}