Rollup merge of #116481 - scottmcm:tweak-combinators, r=cuviper

Reuse existing `Some`s in `Option::(x)or`

LLVM still has trouble re-using discriminants sometimes when rebuilding a two-variant enum, so when we have the correct variant already built, just use it.

That's shorter in the Rust code, as well as simpler in MIR and the optimized LLVM, so might as well: <https://rust.godbolt.org/z/KhdE8eToW>

Thanks to `@veber-alex` for pointing out this opportunity in #101210 (comment)

library/core/src/option.rs

@@ -1475,7 +1475,7 @@ impl<T> Option<T> {
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn or(self, optb: Option<T>) -> Option<T> {
         match self {
-            Some(x) => Some(x),
+            x @ Some(_) => x,
             None => optb,
         }
     }
@@ -1500,7 +1500,7 @@ impl<T> Option<T> {
         F: FnOnce() -> Option<T>,
     {
         match self {
-            Some(x) => Some(x),
+            x @ Some(_) => x,
             None => f(),
         }
     }
@@ -1530,8 +1530,8 @@ impl<T> Option<T> {
     #[stable(feature = "option_xor", since = "1.37.0")]
     pub fn xor(self, optb: Option<T>) -> Option<T> {
         match (self, optb) {
-            (Some(a), None) => Some(a),
-            (None, Some(b)) => Some(b),
+            (a @ Some(_), None) => a,
+            (None, b @ Some(_)) => b,
             _ => None,
         }
     }