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

[clementval]

Summary

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.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Optimizer/CodeGen/CodeGen.cpp

if (isFloatingPointTy(fromTy)) {
       if (toTy.isa<mlir::IntegerType>()) {
         auto fromBits = mlir::LLVM::getPrimitiveTypeSizeInBits(fromTy);
         auto toBits = mlir::LLVM::getPrimitiveTypeSizeInBits(toTy);
         assert(fromBits != toBits);
         if (fromBits > toBits) {
           rewriter.replaceOpWithNewOp<mlir::LLVM::TruncOp>(convert, toTy, op0);
           return mlir::success();
         }
+        if (fromFirTy == i1Type) {
+          rewriter.replaceOpWithNewOp<mlir::LLVM::ZExtOp>(convert, toTy, op0);
+          return mlir::success();
+        }
         rewriter.replaceOpWithNewOp<mlir::LLVM::SExtOp>(convert, toTy, op0);
         return mlir::success();
       }
       // Integer to floating point conversion.
       if (isFloatingPointTy(toTy)) {
         rewriter.replaceOpWithNewOp<mlir::LLVM::SIToFPOp>(convert, toTy, op0);
         return mlir::success();
       }

flang/test/Fir/convert-to-llvm.fir

 // CHECK:         %{{.*}} = llvm.sitofp %[[ARG0]] : i32 to f128
 // CHECK:         %{{.*}} = llvm.trunc %[[ARG0]] : i32 to i1
 // CHECK:         %{{.*}} = llvm.trunc %[[ARG0]] : i32 to i8
 // CHECK:         %{{.*}} = llvm.trunc %[[ARG0]] : i32 to i16
 // CHECK-NOT:     %{{.*}} = llvm.trunc %[[ARG0]] : i32 to i32
 // CHECK:         %{{.*}} = llvm.sext %[[ARG0]] : i32 to i64
 // CHECK:         %{{.*}} = llvm.inttoptr %{{.*}} : i64 to !llvm.ptr<i64>
 
+
+func.func @convert_from_i1(%arg0 : i1) {
+  %0 = fir.convert %arg0 : (i1) -> i32
+  return
+}
+
+// CHECK-LABEL: convert_from_i1(
+// CHECK-SAME: %[[ARG0:.*]]: i1
+// CHECK:        %{{.*}} = llvm.zext %[[ARG0]] : i1 to i32 
+
 // -----
 
 // Test `fir.convert` operation conversion from !fir.ref<> type.
 
 func.func @convert_from_ref(%arg0 : !fir.ref<i32>) {
   %0 = fir.convert %arg0 : (!fir.ref<i32>) -> !fir.ref<i8>
   %1 = fir.convert %arg0 : (!fir.ref<i32>) -> i32
   return