libstdc++: Improve test codegen for interpreting assembly

libstdc++: Improve test codegen for interpreting assembly

In many failure cases it is helpful to inspect the instructions leading
up to the test failure. After this change the location is easier to find
and the branch after failure is easier to find.

libstdc++-v3/ChangeLog:

	* testsuite/experimental/simd/tests/bits/verify.h (verify): Add
	instruction pointer data member. Ensure that the `if (m_failed)`
	branch is always inlined into the calling code. The body of the
	conditional can still be a function call. Move the get_ip call
	into the verify ctor to simplify the ctor calls.
	(COMPARE): Don't mention the use of all_of for reduction of a
	simd_mask. It only distracts from the real issue.