This patch fixes a potential crash due to RegAllocFast not rewriting virtual registers. This essentially happens because of a call to MachineInstr::addRegisterKilled() in the process of allocating a "killed" vreg. The former can eventually delete implicit operands without RegAllocFast noticing, leading to some operands being "skipped" and not rewritten to use physical registers.
Note that I noticed this crash when working on a solution for tying a register with one/multiple of its sub-registers within an instruction. (See problem description here: https://discourse.llvm.org/t/pass-to-tie-an-output-operand-to-a-subregister-of-an-input-operand/67184).
Aside from this fix, I believe there could be further improvements to the RegAllocFast when it comes to instructions with multiple uses of a same virtual register. You can see it in the added test where the implicit uses have been re-written in a somewhat surprising way because of phase ordering. Ultimately, when allocating vregs for an instruction, I believe we should iterate on the vregs it uses (and then process all the operands that use this vregs), instead of directly iterating on operands and somewhat assuming each operand uses a different vreg. This would in the end be quite close to what greedy+virtregrewriter does. If that makes sense, I would probably spin off another patch (after I get more familiar with RegAllocFast).