Ik the diffcount is a lot, but most of it is just boilerplate. It is easier to review on a per-commit basis, and the following commits contain the "interesting" bits (other just soulless adding of const keyword everywhere)

• 7134a49 (modifies the integer reduce_add and reduce_mul intrinsics to use simd_reduce_add_ordered and simd_ordered_mul_ordered, respectively. As integer algebra is associative, it doesn't matter for correctness, and LLVM will use the most performant arrangement. The reason is that rustc_const_eval only implements these ordered reduction intrinsics, unordered reduction seems to be tricky to implement in const_eval)
• fa55d7d (modifies #[simd_test] quite a bit, adds a new "no arguments" mode to ease testing of intrinsics that require no target features and adds support for const testing)
• 391dc70 (rewrites some test helper functions for const)
• b451207 and 1638f94 (contains some modifications to avoid pointer-to-usize casts and for loops)

Small todo: open a tracking issue for x86 const intrinsics