This patch teaches the backend how to lower the FCBRT DAG node to the
vector math library function when using ArmPL. This is similar to what
we already do for llvm.pow/FPOW, however the only way to expose this is
via a DAG combine that converts
FPOW(<2 x double> %x, <2 x double> <double 1.0/3.0, double 1.0/3.0>)
into
FCBRT(<2 x double> %x)
when the appropriate fast math flags are present on the node. I've
updated the DAG combine to handle vector types and only perform the
transformation if there exists a vector library variant of cbrt.
0b36d4265e