Add fixed-non-allocatable operand support (#77)

This allows a non-allocatable `PReg` to be passed on directly to the
allocations vector without any liverange tracking from the register
allocator. The main intended use case is to support ISA-specific special
registers such as a fixed zero register.

fuzz/fuzz_targets/ion_checker.rs

@@ -22,6 +22,7 @@ impl Arbitrary<'_> for TestCase {
                 &Options {
                     reused_inputs: true,
                     fixed_regs: true,
+                    fixed_nonallocatable: true,
                     clobbers: true,
                     control_flow: true,
                     reducible: false,

fuzz/fuzz_targets/ssagen.rs

@@ -23,6 +23,7 @@ impl Arbitrary<'_> for TestCase {
                 &Options {
                     reused_inputs: true,
                     fixed_regs: true,
+                    fixed_nonallocatable: true,
                     clobbers: true,
                     control_flow: true,
                     reducible: false,