Add pass to inject code for robust-buffer-access semantics (#2771)

spirv-opt: Add --graphics-robust-access

Clamps access chain indices so they are always
in bounds.

Assumes:
- Logical addressing mode
- No runtime-array-descriptor-indexing
- No variable pointers

Adds stub code for clamping coordinate and samples
for OpImageTexelPointer.

Adds SinglePassRunAndFail optimizer test fixture.

Android.mk: add source/opt/graphics_robust_access_pass.cpp

Adds Constant::GetSignExtendedValue, Constant::GetZeroExtendedValue