MIR Parser: report an error when a basic block isn't found.

This commit reports an error when the MIR parser can't find
a basic block with the machine basic block's name.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240174 91177308-0d34-0410-b5e6-96231b3b80d8

test/CodeGen/MIR/machine-basic-block-unknown-name.mir

@@ -0,0 +1,18 @@
+# RUN: not llc -start-after branch-folder -stop-after branch-folder -o /dev/null %s 2>&1 | FileCheck %s
+# This test ensures that an error is reported whenever the MIR parser can't find
+# a basic block with the machine basis block's name.
+
+--- |
+
+  define i32 @foo() {
+  entry:
+    ret i32 0
+  }
+
+...
+---
+name:            foo
+body:
+  # CHECK: basic block 'entrie' is not defined in the function 'foo'
+  - name:         entrie
+...