OpExecutionModeId not used unless targeting SPIRV-1.6

It appears that [OpExecutionModeId](https://registry.khronos.org/SPIR-V/specs/unified1/SPIRV.html#OpExecutionModeId) was introduced in SPIR-V 1.2. However, it is not used unless we target SPIR-V 1.6.

Here's a minimal example:

```
#version 450
layout (constant_id = 0) const uint local_size_x = 128;
layout (local_size_x_id = 0, local_size_y = 1, local_size_z = 1) in;
void main() {}
```
When compiled with:
```
glslangValidator.exe -V100 --target-env spirv1.2 -e main repro.comp.glsl -o spirv/repro.comp.spv
```
It produces:
```
; SPIR-V
; Version: 1.2
; Generator: Khronos Glslang Reference Front End; 11
; Bound: 12
; Schema: 0
OpCapability Shader
%1 = OpExtInstImport "GLSL.std.450"
OpMemoryModel Logical GLSL450
OpEntryPoint GLCompute %main "main"
OpExecutionMode %main LocalSize 1 1 1
OpSource GLSL 450
OpName %main "main"
OpName %local_size_x "local_size_x"
OpDecorate %local_size_x SpecId 0
OpDecorate %8 SpecId 0
OpDecorate %gl_WorkGroupSize BuiltIn WorkgroupSize
%void = OpTypeVoid
%3 = OpTypeFunction %void
%uint = OpTypeInt 32 0
%local_size_x = OpSpecConstant %uint 128
%8 = OpSpecConstant %uint 1
%uint_1 = OpConstant %uint 1
%v3uint = OpTypeVector %uint 3
%gl_WorkGroupSize = OpSpecConstantComposite %v3uint %8 %uint_1 %uint_1
%main = OpFunction %void None %3
%5 = OpLabel
OpReturn
OpFunctionEnd
```
But when compiled with:
```
glslangValidator.exe -V100 --target-env spirv1.6 -e main repro.comp.glsl -o spirv/repro.comp.spv
```
Produces:
```
; SPIR-V
; Version: 1.6
; Generator: Khronos Glslang Reference Front End; 11
; Bound: 13
; Schema: 0
OpCapability Shader
%1 = OpExtInstImport "GLSL.std.450"
OpMemoryModel Logical GLSL450
OpEntryPoint GLCompute %main "main"
OpExecutionModeId %main LocalSizeId %7 %uint_1 %uint_1
OpSource GLSL 450
OpName %main "main"
OpName %local_size_x "local_size_x"
OpDecorate %7 SpecId 0
OpDecorate %local_size_x SpecId 0
OpDecorate %10 SpecId 0
%void = OpTypeVoid
%3 = OpTypeFunction %void
%uint = OpTypeInt 32 0
%7 = OpSpecConstant %uint 1
%uint_1 = OpConstant %uint 1
%local_size_x = OpSpecConstant %uint 128
%10 = OpSpecConstant %uint 1
%v3uint = OpTypeVector %uint 3
%12 = OpSpecConstantComposite %v3uint %10 %uint_1 %uint_1
%main = OpFunction %void None %3
%5 = OpLabel
OpReturn
OpFunctionEnd
```

Is the code produced in the first case valid? ExecutionMode is configured as (1, 1, 1), but gl_WorkGroupSize is then defined according to the specialization constants. Is that correct?

If not, should the use of `local_size_x_id` be an error when targeting spir-v versions prior to 1.2?

Start by reproducing the two glslangValidator commands and comparing their generated SPIR-V with the SPIR-V 1.2 rules for OpExecutionModeId and LocalSizeId. Determine whether the SPIR-V 1.2 output is valid and whether local_size_x_id should be rejected for that target; done means the behavior and any required diagnostic are established.

Written by the indexing model from the issue text.