[AMDGPU] adding test for partially masked operands

[AMDGPU] adding test for partially masked operands

This test is testing whether the compiler behaves correctly when only
parts of an operand are masked.
In this case, no optimization is supposed to happen, since neither
the upper nor the lower half is
fully masked. Therefore, none
of the halves can be known to be zero.
The result is a regular multiplication.