[RISCV] More correctly ignore Zfinx register classes in getRegForInlineAsmConstraint.

Until Zfinx is supported in CodeGen we need to convert all Zfinx
register classes to GPR.

Remove the zfinx-types.ll test which didn't test anything meaningful
since -mattr=zfinx isn't implemented completely in llc.

Follow up to D93298.

(cherry picked from commit 6cb42cd6)