Validator: FPRoundingMode decoration (#1482)

This commit checks the following when Shader capability exists:
"The FPRoundingMode decoration can be applied only to a width-only
conversion instruction that is used as the Object operand of an
OpStore storing through a pointer to a 16-bit floating-point object
in the StorageBuffer, Uniform, PushConstant, Input, or Output
Storage Classes.".