[ARM,MVE] Add intrinsics and isel for MVE fused multiply-add.
This adds the ACLE intrinsic family for the VFMA and VFMS
instructions, which perform fused multiply-add on vectors of floats.
I've represented the unpredicated versions in IR using the cross-
platform @llvm.fma IR intrinsic. We already had isel rules to
convert one of those into a vector VFMA in the simplest possible way;
but we didn't have rules to detect a negated argument and turn it into
VFMS, or rules to detect a splat argument and turn it into one of the
two vector/scalar forms of the instruction. Now we have all of those.
The predicated form uses a target-specific intrinsic as usual, but
I've stuck to just one, for a predicated FMA. The subtraction and
splat versions are code-generated by passing an fneg or a splat as one
of its operands, the same way as the unpredicated version.
In arm_mve_defs.h, I've had to introduce a tiny extra piece of
infrastructure: a record id for use in codegen dags which implements
the identity function. (Just because you can't declare a Tablegen
value of type dag which is only a $varname: you have to wrap it
in something. Now I can write (id $varname) to get the same effect.)
Reviewers: dmgreen, MarkMurrayARM, miyuki, ostannard
Reviewed By: dmgreen
Subscribers: kristof.beyls, hiraditya, danielkiss, cfe-commits, llvm-commits
Tags: #clang, #llvm
Differential Revision: https://reviews.llvm.org/D75998