[AArch64] Refactor the Neon vector/scalar floating-point convert intrinsics so
that they use float/double rather than the vector equivalents when appropriate.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196930 91177308-0d34-0410-b5e6-96231b3b80d8
Support: Update documentation for Program functions

The docstrings were describing an older interface that has been replaced with
functions.

Also describe the performance characteristics of FindProgramByName() and
ExecuteAndWait() explaining when it's best to avoid them.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196932 91177308-0d34-0410-b5e6-96231b3b80d8