[SPIR-V]: Fix spirv codegen for uabs intrinsic when argument is unsigned.

[luciechoi]

Fix the spirv codegen for abs() on uint, to not generate OpExtInst instruction but simply load and pass the value. Fixes #7512

Example

[numthreads(1,1,1)]
void main() {
  uint32_t A = uint32_t(0xffffffff);
  uint32_t B = abs(A);
}

Previously generated SPIR-V

 %A = OpVariable %_ptr_Function_uint Function
 %B = OpVariable %_ptr_Function_uint Function
      OpStore %A %uint_4294967295
%13 = OpLoad %uint %A
%14 = OpExtInst %int %1 SAbs %13
      OpStore %B %14

Fix

 %A = OpVariable %_ptr_Function_uint Function
 %B = OpVariable %_ptr_Function_uint Function
      OpStore %A %uint_4294967295
%13 = OpLoad %uint %A
      OpStore %B %13

which is equivalent to

[numthreads(1,1,1)]
void main() {
  uint32_t A = uint32_t(0xffffffff);
  uint32_t B = A;
}

[s-perron]

/AzurePipelines run

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