Expand reduce-style kernels.

Bug: 22631253

Rename the RSForEachExpand pass / source file to RSKernelExpand. In
addition to already being responsible for expanding foreach-style
kernels, RSKernelExpand now expands reduce-style kernels from

  define foo @func(foo %a, foo %b)

into

  define void @func.expand(i8* %inBuf, i8* %outBuf, i32 len)

The expanded function reduces a sub-range of its input.

Change-Id: I518fa9367b57111a2f2a1b23903ff870c421beeb
9 files changed