RFA: Fix match_scratch bug in define_subst

[Joern Rennecke]

Bootstrapped on x86_64-pc-linux-gnu.

2020-12-10  Joern Rennecke  <joern.renne...@embecosm.com>

    Fix bug in the define_subst handling that made match_scratch unusable for
    multi-alternative patterns.

diff --git a/gcc/gensupport.c b/gcc/gensupport.c
index e1ca06dbc1e..4022c661adb 100644
--- a/gcc/gensupport.c
+++ b/gcc/gensupport.c
@@ -1291,6 +1291,9 @@ alter_constraints (rtx pattern, int n_dup, 
constraints_handler_t alter)
     case MATCH_OPERAND:
       XSTR (pattern, 2) = alter (XSTR (pattern, 2), n_dup);
       break;
+    case MATCH_SCRATCH:
+      XSTR (pattern, 1) = alter (XSTR (pattern, 1), n_dup);
+      break;
 
     default:
       break;