[RISCV] Replace sexti32/zexti32 in isel patterns where only one part of their PatFrag...

[RISCV] Replace sexti32/zexti32 in isel patterns where only one part of their PatFrags can match. NFCI

We had an zexti32 after a sign_extend_inreg. The AND X, 0xffffffff
part of the zexti32 should never occur since SimplifyDemandedBits
from the sign_extend_inreg would have removed it.

We also had sexti32 as the root node of a pattern, but SelectionDAGISel
matches assertsext early before the tablegen based patterns are
evaluated.