OpSpecConstantOp doesn't validate itself

[spencer-lunarg]

There is a stale PR attempt here #4907

Basically this (https://godbolt.org/z/v4Yfs91rP)

OpCapability Shader
OpCapability Linkage
OpMemoryModel Logical GLSL450
%uint = OpTypeInt 32 0
%float = OpTypeFloat 32
%uint_1 = OpConstant %uint 1
%float_1 = OpConstant %float 1
%good = OpSpecConstantOp %float QuantizeToF16 %float_1
%bad1 = OpSpecConstantOp %uint QuantizeToF16 %float_1
%bad2 = OpSpecConstantOp %float QuantizeToF16 %uint_1

should throw an error, but doesn't

We need a way for OpSpecConstantOp to be validated