[mlir][StandardToSPIRV] Add support for lowering trunci to SPIR-V to i1 types.

Add a pattern to converting some value to a boolean. spirv.S/UConvert does not
work on i1 types. Thus, the pattern is lowered to cmpi + select.

Reviewed By: antiagainst

Differential Revision: https://reviews.llvm.org/D96851