rustc_codegen_ssa: only create backend BasicBlocks as-needed.

compiler/rustc_codegen_llvm/src/builder.rs

@@ -1148,10 +1148,6 @@ impl BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
         self.cx
     }
 
-    unsafe fn delete_basic_block(&mut self, bb: &'ll BasicBlock) {
-        llvm::LLVMDeleteBasicBlock(bb);
-    }
-
     fn do_not_inline(&mut self, llret: &'ll Value) {
         llvm::Attribute::NoInline.apply_callsite(llvm::AttributePlace::Function, llret);
     }

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -1079,7 +1079,6 @@ extern "C" {
         Fn: &'a Value,
         Name: *const c_char,
     ) -> &'a BasicBlock;
-    pub fn LLVMDeleteBasicBlock(BB: &BasicBlock);
 
     // Operations on instructions
     pub fn LLVMIsAInstruction(Val: &Value) -> Option<&Value>;