[dmd-internals] Three patches for recent regressions

[Don Clugston]

4751 Regression(1.062, 2.047) ICE(constfold.c) >> after error
4753 fail_compilation/fail116.d sends dmd into a loop, exhausting memory
4771 fail_compilation/fail274.d hits a halt in iasm.c

I suggest we introduce a policy for compiler releases: "No regressions
more recent than 2.041/1.057".
We can gradually push this back until all regressions are gone.