spirv-fuzz: Lower probability of adding bit instruction synonyms (#3917)

Fixes #3914.
diff --git a/source/fuzz/fuzzer_context.cpp b/source/fuzz/fuzzer_context.cpp
index 3f0d10e..19d4ca5 100644
--- a/source/fuzz/fuzzer_context.cpp
+++ b/source/fuzz/fuzzer_context.cpp
@@ -31,8 +31,8 @@
 const std::pair<uint32_t, uint32_t> kChanceOfAddingAnotherStructField = {20,
                                                                          90};
 const std::pair<uint32_t, uint32_t> kChanceOfAddingArrayOrStructType = {20, 90};
-const std::pair<uint32_t, uint32_t> kChanceOfAddingBitInstructionSynonym = {20,
-                                                                            90};
+const std::pair<uint32_t, uint32_t> kChanceOfAddingBitInstructionSynonym = {5,
+                                                                            20};
 const std::pair<uint32_t, uint32_t>
     kChanceOfAddingBothBranchesWhenReplacingOpSelect = {40, 60};
 const std::pair<uint32_t, uint32_t> kChanceOfAddingCompositeInsert = {20, 50};