x64: Compare source arguments in produces_const

[elliottt]

This PR changes the behavior of produces_const in the x64 backend to check that the two source registers are the same, rather than check that the second source and destination are the same. The reason for this change is a mismatch in behavior between the ISLE constructors for those pseudo instructions and the existing rust functions for constructing them: the rust functions always ensure that the first source and destination registers are the same, while the isle functions introduce a fresh destination register and take the two sources as arguments.

This change preserves the existing behavior for instructions constructed through the rust functions, but allows the lower.isle to use instructions like (x64_xor x x) where x is a fresh register without causing a panic in RA2.

[cfallin]

Thanks!

To clarify the impact of this to anyone reading: there is no miscompilation currently, because all producers of these instruction variants set src1 == dst. But it becomes an issue once one starts to produce instructions where this is not true (i.e., actually leveraging the ability to generate SSA-like VCode), which @elliottt's in-progress work is doing. It's still clearly a bug so this fix makes sense on its own :-)