[flang] Use llvm.zext when converting from i1 -> iXX

CodeGen used llvm.sext when converting fir.convert %0 : (i1) -> iXX
where iXX is any integer. This leads to wrong values when the initial
i1 is equal to 1.

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D145984