target/arm: Define and use new write_fp_*reg_merging() functions

target/arm: Define and use new write_fp_*reg_merging() functions

For FEAT_AFP's FPCR.NEP bit, we need to programmatically change the
behaviour of the writeback of the result for most SIMD scalar
operations, so that instead of zeroing the upper part of the result
register it merges the upper elements from one of the input
registers.

Provide new functions write_fp_*reg_merging() which can be used
instead of the existing write_fp_*reg() functions when we want this
"merge the result with one of the input registers if FPCR.NEP is
enabled" handling, and use them in do_fp3_scalar_with_fpsttype().

Note that (as documented in the description of the FPCR.NEP bit)
which input register to use as the merge source varies by
instruction: for these 2-input scalar operations, the comparison
instructions take from Rm, not Rn.

We'll extend this to also provide the merging behaviour for
the remaining scalar insns in subsequent commits.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>