[RISCV][NFC] Add tests for SLP vectorization of math functions

RISCV has "vfabs.v" and "vfsqrt.v" so math functions abs and sqrt
can be SLP vectorized. But others exp/log/sin/asin/sinh/asinh/...
can not.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D145562