transform broken MIR warnings to hard ICEs
transform broken MIR warnings to hard ICEs

We ought to do that sometime, and this PR fixes all broken MIR errors I
could find.

graydon/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust