Further fixes to lossless_cast. Also ensure it returns a value
properly casted to the target type.

Add simd_op_check cases, including one that doesn't work yet.
Merge branch 'master' of https://github.com/halide/Halide
Remove apps/HelloAndroid/jni/halide

Compilation by-product that should not have been checked in to source
control
Promote casts inside lets for peephole matching