[llvm-exegesis] Improve documentation.

Summary:
- Better flag names.
- Fix flag reference in doc.
- Add usage examples in doc.

Fixes PR37497.

Reviewers: gchatelet

Subscribers: llvm-commits, tschuett

Differential Revision: https://reviews.llvm.org/D47015

llvm-svn: 332708
diff --git a/llvm/tools/llvm-exegesis/llvm-exegesis.cpp b/llvm/tools/llvm-exegesis/llvm-exegesis.cpp
index 90b2ebb..3c57706 100644
--- a/llvm/tools/llvm-exegesis/llvm-exegesis.cpp
+++ b/llvm/tools/llvm-exegesis/llvm-exegesis.cpp
@@ -49,7 +49,7 @@
 
 enum class BenchmarkModeE { Latency, Uops, Analysis };
 static llvm::cl::opt<BenchmarkModeE> BenchmarkMode(
-    "benchmark-mode", llvm::cl::desc("the benchmark mode to run"),
+    "mode", llvm::cl::desc("the mode to run"),
     llvm::cl::values(
         clEnumValN(BenchmarkModeE::Latency, "latency", "Instruction Latency"),
         clEnumValN(BenchmarkModeE::Uops, "uops", "Uop Decomposition"),