The first source has the same EEW as the destination, but we're
using earlyclobber which prevents them from ever being the same
register. This patch attempts to work around this.
-For unmasked .wv, add a special TIED pseudo that pretends like
the first operand and the destination must be the same register. This
disables the earlyclobber for that source. Mark the instruction
as convertible to 3 address form which will switch it to the
original untied pseudo when the TwoAddressInstructionPass decides
that keeping them tied would require an extra copy. This uses
code in RISCVInstrInfo.cpp to do the conversion to the untied
The untie test case show that we can generate the untied version.
Not sure it was profitable to do it in this case, but they have
really simple IR.