[RISCV] Remove vmv.s.x and vmv.x.s lmul pseudo variants (#71501)

vmv.s.x and vmv.x.s ignore LMUL, so we can replace the PseudoVMV_S_X_MX
and
PseudoVMV_X_S_MX with just one pseudo each. These pseudos use the VR
register
class (just like the actual instruction), so we now only have TableGen
patterns for vectors of LMUL <= 1.
We now rely on the existing combines that shrink LMUL down to 1 for
vmv_s_x_vl (and vfmv_s_f_vl). We could look into removing these combines
later and just inserting the nodes with the correct type in a later
patch.

The test diff is due to the fact that a PseudoVMV_S_X/PsuedoVMV_X_S no
longer
carries any information about LMUL, so if it's the only vector pseudo
instruction in a block then it now defaults to LMUL=1.