Fix some bugs in fast dfa's borrow checker logic

[rikkimax]

Thanks to @ntrel for finding these bugs!

[github-actions]

DMD perf check

Metric	Base	PR	Δ
compile dmd itself (wall)	12.4 s	12.6 s	+2.14%

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)	5,123.5 M	5,123.2 M	-0.006%
compile Phobos codegen (instr)	1,472.6 M	1,472.5 M	-0.010%
compile vibe.d (instr)	15,129.9 M	15,129.8 M	-0.001%
dmd binary size (stripped)	6.86 MB	6.87 MB	+0.06%
hello binary size (stripped)	0.72 MB	0.72 MB	0.00%
peak RSS (compile hello.d)	43.42 MB	43.41 MB	-0.02%
peak RSS (compile Phobos)	618.7 MB	617.4 MB	-0.20%
peak RSS (compile vibe.d)	1918 MB	1917 MB	-0.06%
compile dmd itself (wall)	12.4 s	12.6 s	+2.14%
compile hello.d (wall)	66.7 ms	65.7 ms	-1.52%
compile Phobos (wall)	1,581 ms	1,600 ms	+1.20%

_{734bc18 vs merge-base 1906158 · about these metrics}