[NFC] Add MutableSpan bounds check test

Author: meg-gupta

test/SILOptimizer/mutable_span_stdlib_bounds_check_tests.swift

@@ -0,0 +1,31 @@
+// RUN: %target-swift-frontend -O -emit-sil %s -disable-availability-checking | %FileCheck %s --check-prefix=CHECK-SIL 
+
+public protocol P {
+   mutating func mutate(_ other: Self)
+}
+
+// CHECK-SIL-LABEL: sil @$s38mutable_span_stdlib_bounds_check_tests0a1_B7_doubleyys11MutableSpanVyxGzAA1PRzlF :
+// CHECK-SIL: bb3({{.*}}):
+// CHECK-SIL-NOT: end_cow_mutation
+// CHECK-SIL-NOT: cond_fail "index out of bounds"
+// CHECK-SIL-LABEL: } // end sil function '$s38mutable_span_stdlib_bounds_check_tests0a1_B7_doubleyys11MutableSpanVyxGzAA1PRzlF'
+public func mutable_span_double<T: P>(_ ms: inout MutableSpan<T>) {
+  for i in ms.indices {
+    ms[i].mutate(ms[i])
+  }
+}
+
+extension Int : P {
+  public mutating func mutate(_ other: Int) {
+    self += other
+  }
+}
+
+// CHECK-SIL-LABEL: sil @$s38mutable_span_stdlib_bounds_check_tests17specializedCalleryySaySiGzF : $@convention(thin) (@inout Array<Int>) -> () {
+// CHECK-SIL-NOT: cond_fail "index out of bounds"
+// CHECK-SIL-LABEL: } // end sil function '$s38mutable_span_stdlib_bounds_check_tests17specializedCalleryySaySiGzF'
+public func specializedCaller(_ array: inout Array<Int>) {
+  var mut = array.mutableSpan
+  mutable_span_double(&mut)
+}
+