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