spirv_assembly: Don't use booleans in SSBOs

spirv_assembly: Don't use booleans in SSBOs

The bool type in spirv isn't allowed in external storage.  From the SPIR-V
spec for OpTypeBool:

    "Values of this type can only be either true or false. There is no
    physical size or bit pattern defined for these values. If they are
    stored (in conjunction with OpVariable), they can only be used with
    logical addressing operations, not physical, and only with
    non-externally visible shader Storage Classes: Workgroup,
    CrossWorkgroup, Private, and Function."

Instead, we should use an integer and use OpSelect to convert from a
boolean to the expected 0/1 value the test wants.