Use cl_uint in some SPIR-V tests to avoid UB (#2159)

The following tests under `spirv_new`:
* `op_branch_conditional`
* `op_phi`
* `op_selection_merge`

were using randomly generated `cl_int` values, and storing the
difference between them as a `cl_int`. If one of the generated values is
negative and large enough, while the other is positive and large enough,
the difference then becomes a larger value that cannot be represented
using `cl_int`.

Switch the tests to use `cl_uint` instead, and update the relevant
spvasm{32,64} files to use `OpULessThan` instead of `OpSLessThan`.

Signed-off-by: Ahmed Hesham <ahmed.hesham@arm.com>

test_conformance/spirv_new/spirv_asm/branch_conditional.spvasm64

@@ -45,7 +45,7 @@
          %26 = OpInBoundsPtrAccessChain %_ptr_CrossWorkgroup_uint %rhs %23
          %27 = OpLoad %uint %26 Aligned 4
                OpStore %19 %uint_0
-         %28 = OpSLessThan %bool %25 %27
+         %28 = OpULessThan %bool %25 %27
                OpBranchConditional %28 %29 %30
          %29 = OpLabel
          %31 = OpISub %uint %27 %25