SPIRV: Add image instruction support and atomic counter support.

the expectations of old GLSL shadow*() lookups.

Structured control-flow rules allow leaving the middle of a construct through
a return, but not through a jump to a block that does a return.

Addresses issue #58.

This generally simplifies access chain generation, with far fewer  type conversions.
It is particularly important to future SPIR-V changes where there is less aggregate
type uniqueness due to carrying different layout information with the type.

This includes run-time block.member.length() (OpArrayLength).

Front-end Arrays of arrays: Add ES-specific checks and tests.  AoA should be quite close to done now.

Sync local master branch from the upstream

SPIR-V: Add support for texelFetch

SPIRV: Decorate matrices and arrays with their strides

cmake: Don't override CMAKE_INSTALL_PREFIX

Preprocessor:  'defined' cannot be used as a macro name in #define or #undef.

There's nothing really special about texelFetch other than the opcode and
some restrictions so adding it is pretty trivial.

Stop forcing CMAKE_INSTALL_PREFIX="install". If the user manually set
CMAKE_INSTALL_PREFIX, then trust that he knows what he's doing.

This patch does NOT change the project's default value ("install") of
CMAKE_INSTALL_PREFIX.

Change-Id: I81b46dd1986427b498fe6316bed03f01689987d4