Added little comment to each test.

Author: stefanp-synopsys

llvm/test/CodeGen/PowerPC/subreg-coalescer.mir

@@ -2,6 +2,10 @@
 # RUN: llc -mtriple powerpc64le-unknown-linux-gnu -mcpu=pwr8 -x mir < %s \
 # RUN:   -verify-coalescing --run-pass=register-coalescer -o - | FileCheck %s
 
+# Check that the register coalescer correctly handles merging live ranges over
+# SUBREG_TO_REG on PowerPC. The -verify-coalescing option will give an error if
+# this is incorrect.
+
 ---
 name: check_subregs
 alignment:       16

llvm/test/CodeGen/X86/subreg-fail.mir

@@ -3,6 +3,10 @@
 # RUN:   -verify-coalescing -enable-subreg-liveness=true \
 # RUN:   --run-pass=register-coalescer -o - | FileCheck %s
 
+# Check that the register coalescer correctly handles merging live ranges over
+# SUBREG_TO_REG on X86. The -verify-coalescing option will give an error if
+# this is incorrect.
+
 ---
 name:            test1
 alignment:       16