[RISCV] Fix using undefined variable %pt2 in mask-reg-alloc.mir testcase (#70764)

First PseudoVMERGE_VIM_M1 should use %pt1 as its operand instead of
%pt2.

I found this error when I add LiveIntervals analysis pass in my
downstream. And it crashes with the message:

```
Use of %7 does not have a corresponding definition on every path:
112r %6:vrnov0 = PseudoVMERGE_VIM_M1 %pt2:vrnov0(tied-def 0), %2:vr, 1, %4:vmv0, 1, 3
LLVM ERROR: Use not jointly dominated by defs.
```