Update double-precision test configurations
These changes improve coverage of double precision, using more release
mode, particularly with latest gcc and icc, and using 128-bit SIMD,
which have been cases that were buggy recently. The other aspects of
the configurations that have been modified have been
non-critical. Where appropriate, brief rationales are recorded. This
resolves an old TODO item in the post-submit matrix.
Fixed a sign mismatch in initializing an OpenCL variable that didn't
need to be initialized.
Noted relevant new TODOs.
Refs #2300, #2325, #2326, #2334, #2335, #2336, #2337, #2338
Change-Id: I131fa1a6776d1e7809799c3f931a1fc8100fcdc9