[RISCV] Improve register allocation for masked vwadd(u).wv, vwsub(u).wv, vfwadd.wv, and vfwsub.wv.

The first source has the same EEW as the destination, but we're
using earlyclobber which prevents them from ever being the same
register.

To workaround this, add a special TIED pseudo to use whenever the
first source and merge operand are the same value. This allows
us to use a single operand for the merge operand and first source
which we can then tie to the destination. A tied source disables
earlyclobber for that operand.

Reviewed By: arcbbb

Differential Revision: https://reviews.llvm.org/D103211