[arm64] Constant rematerialization fix for loads clobbering a constant register #479
During rematerialization, the register allocator is unable to see when the write to a destination register from a register holding a constant overwrites the register and incorrectly uses it for rematerialization, thus causing a crash.
The first patch in this PR creates a new rset that keeps track of such constant regs and adds a new register allocator entry point ra_alloc_rematsafe() that marks the register as unsafe if it holds a constant and is reused for the destination. The arm64 backend uses this entry point and the new rset (rematset) to make sure that it does not use an unsafe constant register for rematerialization either during an ra_allock or in emit_kdelta.
PR #438 describes an issue with constant rematerialization where a rematerializaed constant could use a register it thought was constant but was in fact clobbered in the same instruction that it was defined in. Fix the problem by adding a mask and a safe entry point for the allocator that marks constant holding registers that are unsafe for rematerialization due to them being the same as the destination register for an instruction it is used in. While this is only implemented in asm_fusexref, it should ideally be implemented for all instructions where the destination is likely to clobber one of the source registers and if one or more of the source registers hold constants.