[SDAG] Revert r296476 (and r296486, r296668, r296690).

This patch causes compile times for some patterns to explode. I have
a (large, unreduced) test case that slows down by more than 20x and
several test cases slow down by 2x. I'm sending some of the test cases
directly to Nirav and following up with more details in the review log,
but this should unblock anyone else hitting this.

llvm-svn: 296862
diff --git a/llvm/test/CodeGen/Mips/o32_cc_vararg.ll b/llvm/test/CodeGen/Mips/o32_cc_vararg.ll
index 73aad48..1dc3a92 100644
--- a/llvm/test/CodeGen/Mips/o32_cc_vararg.ll
+++ b/llvm/test/CodeGen/Mips/o32_cc_vararg.ll
@@ -29,9 +29,9 @@
 
 ; CHECK-LABEL: va1:
 ; CHECK: addiu   $sp, $sp, -16
-; CHECK: sw      $5, 20($sp)
 ; CHECK: sw      $7, 28($sp)
 ; CHECK: sw      $6, 24($sp)
+; CHECK: sw      $5, 20($sp)
 ; CHECK: lw      $2, 20($sp)
 }
 
@@ -83,8 +83,8 @@
 
 ; CHECK-LABEL: va3:
 ; CHECK: addiu   $sp, $sp, -16
-; CHECK: sw      $6, 24($sp)
 ; CHECK: sw      $7, 28($sp)
+; CHECK: sw      $6, 24($sp)
 ; CHECK: lw      $2, 24($sp)
 }