Some intrinsics need to be invoked with constant arguments to get the right
instruction to get generated, so this commit enhances the `assert_instr` macro
to enable this ability. Namely you pass constant arguments like:
#[assert_instr(foo, a = b)]
where this will assert that the intrinsic, when invoked with argument `a` equal
to the value `b` and all other arguments passed from the outside, will generate
the instruction `foo`.
Closes#49
This commit alters the test suite to unconditionally compile and run all tests,
regardless of the ambient target features enabled. This then uses a new
convenience macro, `#[simd_test]`, to guard all tests with the appropriate
`cfg_feature_enabled!` and also enable the `#[target_feature]` appropriately.
