[ExecutionTests] Fix shader source to prevent OOB access for ExecutionTests::AtomicsShared64Test

[alsepkow]

Fixes #5198.

ExecutionTests::AtomicsShared64Test could sporadically fail.

g_sint64Share is indexed with ix%3 + 1, which could lead to an out of bounds access.
Increase the size of g_sint64Share to 4 and ensure all values are initialized.

[alsepkow]

I lost access to a machine with a reliable repro. But I did validate that the test still passes with this change on another machine with the updated shader source.

[tex3d]

Do you think we could fix the same issue under AtomicsHeap as well? It has:

DirectXShaderCompiler/tools/clang/unittests/HLSLExec/ShaderOpArith.xml

Lines 2424 to 2426 in 68dedee

	groupshared uint g_uintShare[12];
	groupshared int g_sintShare[6];
	groupshared uint g_xchgShare[128];

DirectXShaderCompiler/tools/clang/unittests/HLSLExec/ShaderOpArith.xml

Lines 2553 to 2560 in 68dedee

	void InitSharedMem(uint ix) {
	// Zero-init shared memory, with special cases
	if (ix < 6)
	g_uintShare[ix] = ix == 1 ? 99999999 : ix == 3 ? -1 : 0;
	if (ix < 3)
	g_sintShare[ix] = ix == 1 ? 99999999 : 0;
	if (ix < 64)
	g_xchgShare[ix] = 0;

I'm not even sure why the groupshared arrays were doubled in size there, since it doesn't initialize or use the second half of g_uintShare or g_xchgShare. For g_sintShare, it just uses one extra element (though it could have indexed differently for g_sintShare as there really are only 2 elements used).