[AArch64] Add tablegen patterns for faddp of two extracts

This adds some simple tablegen patterns for converting
`faddp v2f32 extractlow(Rn), v2f32 extracthigh(Rn)` to
`faddp v4f32 Rn, v4f32 Rn` using the q variants of the
instructions, avoiding the extra ext needed to extract
the high lanes. Only the bottom lanes of the new faddp
are used, the second Rn operand is used as a placeholder.
It uses Rn to prevent any false dependencies, but could
equally by undef.

Differential Revision: https://reviews.llvm.org/D152245