Fix SPIR-V compilation error with arrays of samplers.

This CL addresses the root cause of the fuzzer issue, by checking for
LayoutIsSupported before getting the MemoryLayout of a type. However,
this array ought to be detected as an error everywhere, as samplers are
opaque types; at present, this code compiles without error in GLSL and
Metal. This is an issue for followup CLs.

GLSL's actual support for arrays of samplers is interesting and probably
too nuanced for us to try to emulate:

https://www.khronos.org/opengl/wiki/Data_Type_(GLSL)#Opaque_arrays

"Under GLSL version 3.30, Sampler arrays (the only opaque type 3.30
provides) can be declared, but they can only be accessed by compile-time
integral Constant Expressions. So you cannot loop over an array of
samplers, no matter what the array initializer, offset and comparison
expressions are.

Under GLSL 4.00 and above, array indices leading to an opaque value can
be accessed by non-compile-time constants, but these index values must
be dynamically uniform. The value of those indices must be the same
value, in the same execution order, regardless of any non-uniform
parameter values, for all shader invocations in the invocation group."

Change-Id: Ib382f5c3b563f996b3c8f1eb6b021b6d31fa9ce7
Bug: oss-fuzz:28107
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/339159
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
6 files changed