Add file-check tests for bounds-check optimizations on Span

And apply the unsigned-compare optimization one other place that I missed the first time, as revealed by the new tests.

Author: stephentyrone

test/stdlib/Span/BoundsCheckOptimization.swift

@@ -0,0 +1,61 @@
+//===--- BoundsCheckOptimization.swift ------------------------*- swift -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+// RUN: %target-swift-frontend -primary-file %s -O -emit-assembly | %FileCheck %s --check-prefix CHECK --check-prefix CHECK-%target-cpu
+
+import Swift
+
+func read(index: Int, span: Span<UInt8>) -> UInt8 {
+  span[index]
+}
+// CHECK: s22BoundsCheckElimination4read5index4spans5UInt8VSi_s4SpanVyAFGtF:
+
+// CHECK-arm64-NOT: tbnz
+// CHECK-arm64: cmp
+// all unsigned comparison spellings?
+// CHECK-arm64-NEXT: b.{{hs|hi|ls|lo|cc|cs}}
+// CHECK-arm64-NEXT: ldrb
+// CHECK-arm64-NEXT: ret
+
+// CHECK-x86_64-NOT: test
+// CHECK-x86_64: cmp
+// all unsigned comparison spellings?
+// CHECK-x86_64-NEXT: j{{a|ae|b|be|c|na|nae|nb|nbe|nc}}
+// CHECK-x86_64-NEXT: movzb
+// x86_64 might have a frame pointer operation before ret
+
+func write(value: UInt8, index: Int, span: inout MutableSpan<UInt8>) {
+  span[index] = value
+}
+// CHECK: s22BoundsCheckElimination5write5value5index4spanys5UInt8V_Sis11MutableSpanVyAGGztF:
+
+// CHECK-arm64-NOT: tbnz
+// CHECK-arm64: cmp
+// all unsigned comparison spellings?
+// CHECK-arm64-NEXT: b.{{hs|hi|ls|lo|cc|cs}}
+// no second compare
+// CHECK-arm64-NOT: cmp
+// no test after compare
+// CHECK-arm64-NOT: tbnz
+// CHECK-arm64: strb
+// CHECK-arm64-NEXT: ret
+
+// CHECK-x86_64-NOT: test
+// CHECK-x86_64: cmp
+// all unsigned comparison spellings?
+// CHECK-x86_64-NEXT: j{{a|ae|b|be|c|na|nae|nb|nbe|nc}}
+// no second compare
+// CHECK-x86_64-NOT: cmp
+// no test after compare
+// CHECK-x86_64-NOT: test
+// CHECK-x86_64: movb
+// x86_64 might have a frame pointer operation before ret