[DXIL][Analysis] Make sure resource accessors are contiguous

When some resource types were present, but not all of them, we were
ending up in a situation where we would fail to initialize the `FirstX`
variables and get incorrect iterators.

Fixes #128560.

Author: bogner

llvm/lib/Analysis/DXILResource.cpp

@@ -756,6 +756,11 @@ void DXILBindingMap::populate(Module &M, DXILResourceTypeMap &DRTM) {
       NextID = 0;
     }
 
+    // We need to make sure the types of resource are ordered even if some are
+    // missing.
+    FirstCBuffer = std::min({FirstCBuffer, FirstSampler});
+    FirstUAV = std::min({FirstUAV, FirstCBuffer});
+
     // Adjust the resource binding to use the next ID.
     RBI.setBindingID(NextID++);
   }

llvm/test/CodeGen/DirectX/Metadata/cbuffer-only.ll

@@ -0,0 +1,19 @@
+; Regression test for https://github.com/llvm/llvm-project/issues/128560 -
+; check that cbuffers are populated correctly when there aren't any other kinds
+; of resource.
+
+; RUN: opt -S -passes=dxil-translate-metadata %s | FileCheck %s
+
+target triple = "dxil-pc-shadermodel6.6-compute"
+
+define void @cbuffer_is_only_binding() {
+  %cbuf = call target("dx.CBuffer", target("dx.Layout", {float}, 4, 0))
+      @llvm.dx.resource.handlefrombinding(i32 1, i32 8, i32 1, i32 0, i1 false)
+  ; CHECK: %cbuffer = type
+
+  ret void
+}
+
+; CHECK:      @[[CB0:.*]] = external constant %cbuffer
+
+; CHECK: !{i32 0, ptr @[[CB0]], !""