Allow conditional lit tests in Subzero, based on build flags.

tests_lit/reader_tests/binops.ll

 ; Tests if we can read binary operators.
 
 ; RUN: %p2i -i %s --insts | FileCheck %s
+; RUN: %l2i -i %s --insts | %ifl FileCheck %s
+; RUN: %lc2i -i %s --insts | %iflc FileCheck %s
 
 ; TODO(kschimpf): add i8/i16. Needs bitcasts.
 
 define i32 @AddI32(i32 %a, i32 %b) {
 entry:
   %add = add i32 %b, %a
   ret i32 %add
 }
 
 ; CHECK:      define i32 @AddI32(i32 %a, i32 %b) {
@@ skipping 942 matching lines @@
 entry:
   %lshr = lshr <4 x i32> %b, %a
   ret <4 x i32> %lshr
 }
 
 ; CHECK-NEXT: define <4 x i32> @LshrV4I32(<4 x i32> %a, <4 x i32> %b) {
 ; CHECK-NEXT: entry:
 ; CHECK-NEXT:   %lshr = lshr <4 x i32> %b, %a
 ; CHECK-NEXT:   ret <4 x i32> %lshr
 ; CHECK-NEXT: }