reg,pass: refactor allocation of aliased registers (#121)

Issue #100 demonstrated that register allocation for aliased registers is
fundamentally broken. The root of the issue is that currently accesses to the
same virtual register with different masks are treated as different registers.
This PR takes a different approach:

* Liveness analysis is masked: we now properly consider which parts of a register are live
* Register allocation produces a mapping from virtual to physical ID, and aliasing is applied later

In addition, a new pass ZeroExtend32BitOutputs accounts for the fact that 32-bit writes in 64-bit mode should actually be treated as 64-bit writes (the result is zero-extended).

Closes #100

build/global.go

@@ -70,7 +70,13 @@ func Constraint(t buildtags.ConstraintConvertable) { ctx.Constraint(t) }
 // constraint comments.
 func ConstraintExpr(expr string) { ctx.ConstraintExpr(expr) }
 
-// GP8 allocates and returns a general-purpose 8-bit register.
+// GP8L allocates and returns a general-purpose 8-bit register (low byte).
+func GP8L() reg.GPVirtual { return ctx.GP8L() }
+
+// GP8H allocates and returns a general-purpose 8-bit register (high byte).
+func GP8H() reg.GPVirtual { return ctx.GP8H() }
+
+// GP8 allocates and returns a general-purpose 8-bit register (low byte).
 func GP8() reg.GPVirtual { return ctx.GP8() }
 
 // GP16 allocates and returns a general-purpose 16-bit register.