[Lexicon] Add BDCE

Summary: Add an entry to the Lexicon for "BDCE."

Reviewers: jmolloy, hfinkel

Reviewed By: jmolloy

Differential Revision: https://reviews.llvm.org/D31861

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@302169 91177308-0d34-0410-b5e6-96231b3b80d8

docs/Lexicon.rst

@@ -38,6 +38,13 @@ B
 **BB Vectorization**
     Basic-Block Vectorization
 
+**BDCE**
+    Bit-tracking dead code elimination. Some bit-wise instructions (shifts,
+    ands, ors, etc.) "kill" some of their input bits -- that is, they make it
+    such that those bits can be either zero or one without affecting control or
+    data flow of a program. The BDCE pass removes instructions that only
+    compute these dead bits.
+
 **BURS**
     Bottom Up Rewriting System --- A method of instruction selection for code
     generation.  An example is the `BURG