Monomorphize a type argument of size-of operation during codegen

This wasn't necessary until MIR inliner started to consider drop glue as
a candidate for inlining; introducing for the first time a generic use
of size-of operation.

No test at this point since this only happens with a custom inlining
threshold.

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

@@ -502,6 +502,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             }
 
             mir::Rvalue::NullaryOp(mir::NullOp::SizeOf, ty) => {
+                let ty = self.monomorphize(&ty);
                 assert!(bx.cx().type_is_sized(ty));
                 let val = bx.cx().const_usize(bx.cx().layout_of(ty).size.bytes());
                 let tcx = self.cx.tcx();