ARM NEON: add vcvtX (with rounding mode) intrinsics to v8 ARM.

These instructions (well, the f32 ones) are supported on 32-bit ARMv8, not just
AArch64. Now that the arm_neon.td refactoring is complete, adding them is
surprisingly simple.

rdar://problem/16035743

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201661 91177308-0d34-0410-b5e6-96231b3b80d8