[SPIR-V][vk::SampledTexture] #6. Add remaining .Gather...() methods for vk::SampledTexture2D type.

[luciechoi]

Part of #7979

TemplateType GatherRed(float2 Location, int2 Offset1, int2 Offset2, int2 Offset3, int2 Offset4, uint Status);
TemplateType GatherGreen(float2 Location, int2 Offset1, int2 Offset2, int2 Offset3, int2 Offset4, uint Status);
TemplateType GatherBlue(float2 Location, int2 Offset1, int2 Offset2, int2 Offset3, int2 Offset4, uint Status);
TemplateType GatherAlpha(float2 Location, int2 Offset1, int2 Offset2, int2 Offset3, int2 Offset4, uint Status);
TemplateType GatherCmp(float Location, float CompareValue, int2 Offset, uint Status);
TemplateType GatherCmpRed(float Location, float CompareValue, int2 Offset1, int2 Offset2, int2 Offset3, int2 Offset4, uint Status);

GatherCmpGreen, GatherCmpBlue, GatherCmpAlpha, GatherRaw are not supported.

[Keenuts]

LGTM for the Gather commit (parent commit in another PR has some comments)