SEV replacement will not work as expected about llvm-mos

[johnwbyrd]

See also the 65C02's TSB, TRB, BBS, and BBR instructions.

[mysterymath]

For the use of $a, LLVM has an undef flag to mark a register as "used, but not really". I forgot to add this flag to this use of $a; I'll correct this shortly.

Regarding N and Z, I realized that so many instructions clobber them, it's not really worth modelling them explicitly except for the instructions that directly deal with them. Accordingly, we maintain as an invariant that N and Z cannot contain live values outside of certain tightly bundled patterns. Broadly, this allows us to emit pseudoinstructions without worrying about N and Z. This makes dealing with N and Z more complicated, but it's better to make something relatively rare complicated (N/Z usage) over something relatively common (basically every pseudoinstruction). I was able to take this invariant far enough that instructions no longer even bear "implicit-def $nz"; the fact that instructions clobber N and Z isn't needed by the compiler anymore, since the invariant picks up the slack.

[mysterymath]

Fixed with 2da8c05