Optimization issue when specifying --target-env

Hello, I noticed a performance degradations when calling some heavy non-side-effected GLSL function a few times, without using its result. Simplifying the code and checking that issue still exists, I ended up with this code:

```GLSL
#version 450
#extension GL_ARB_separate_shader_objects : enable
#extension GL_EXT_nonuniform_qualifier : enable

struct SampleStruct {
int field;
};

layout (set = 0, binding = 1) uniform GlobalDSet0 {
SampleStruct[16768] elems;
} buffers[];

SampleStruct read_buf() { return buffers[0].elems[0]; }

void main() {
read_buf();
}
```

Which compiles (`glslc shader.frag -O --target-env=vulkan1.2`) into:
```
; SPIR-V
; Version: 1.5
; Generator: Google Shaderc over Glslang; 11
; Bound: 37
; Schema: 0
OpCapability Shader
%1 = OpExtInstImport "GLSL.std.450"
OpMemoryModel Logical GLSL450
OpEntryPoint Fragment %4 "main" %19
OpExecutionMode %4 OriginUpperLeft
OpMemberDecorate %_struct_11 0 Offset 0
OpDecorate %_arr__struct_11_uint_16768 ArrayStride 16
OpMemberDecorate %_struct_15 0 Offset 0
OpDecorate %_struct_15 Block
OpDecorate %19 DescriptorSet 0
OpDecorate %19 Binding 1
%void = OpTypeVoid
%3 = OpTypeFunction %void
%int = OpTypeInt 32 1
%_struct_7 = OpTypeStruct %int
%_struct_11 = OpTypeStruct %int
%uint = OpTypeInt 32 0
%uint_16768 = OpConstant %uint 16768
%_arr__struct_11_uint_16768 = OpTypeArray %_struct_11 %uint_16768
%_struct_15 = OpTypeStruct %_arr__struct_11_uint_16768
%uint_1 = OpConstant %uint 1
%_arr__struct_15_uint_1 = OpTypeArray %_struct_15 %uint_1
%_ptr_Uniform__arr__struct_15_uint_1 = OpTypePointer Uniform %_arr__struct_15_uint_1
%19 = OpVariable %_ptr_Uniform__arr__struct_15_uint_1 Uniform
%int_0 = OpConstant %int 0
%_ptr_Uniform__struct_11 = OpTypePointer Uniform %_struct_11
%4 = OpFunction %void None %3
%5 = OpLabel
%34 = OpAccessChain %_ptr_Uniform__struct_11 %19 %int_0 %int_0 %int_0
%35 = OpLoad %_struct_11 %34
%36 = OpCopyLogical %_struct_7 %35
OpReturn
OpFunctionEnd
```

As I can see, `read_buf()` is a pure function, but calling it generates additional code in `main()`.

This issue occures when I set `--target-env` to `vulkan1.2` or `vulkan1.3` , but clears out at `vulkan1.0`, `vulkan1.1`, or without specifying target env at all.

Version:
glslc from VulkanSDK 1.3.268.0
```
$ glslc --version
shaderc v2023.7 v2023.7
spirv-tools v2023.5 v2022.4-352-g360d469b
glslang 11.1.0-803-g48f9ed8b

Target: SPIR-V 1.0
```

Am I missing something about these Vulkan versions, or is this an optimizer bug?