[llvm-extract] Update the help message for group extraction feature

[jsji]

Summary

https://reviews.llvm.org/D60973 exposed the group extraction feature of
the BlockExtractor to llvm-extract.
However, the help message was not updated, so users might not be able to
know how to use this feature without looking into history/commits.

This patch just update the help message to show how to use this group
extraction feature.

[qcolombet]

Hi Jinsong,

Good catch.

Comment below.

Cheers,
-Quentin

llvm/tools/llvm-extract/llvm-extract.cpp

78

I find the use of "group" here confusing.
Maybe we should stick to <function, basic block1[;basic block2...]> like you did in the value_desc.

Maybe also specify that func:bb1;bb2 is not equivalent to func:bb1 func:bb2 (1 function created with bb1 + bb2, vs. two function created; one with bb1, one with bb2).

[jsji]

Agree, how about this?

[jsji]

Add missing '-'.

[qcolombet]

llvm/tools/llvm-extract/llvm-extract.cpp

84

I think extract is more appropriate than split.

[jsji]

Use 'extract' instead of 'split'.

llvm/tools/llvm-extract/llvm-extract.cpp

static cl::list<std::string>
     ExtractRegExpFuncs("rfunc",
                        cl::desc("Specify function(s) to extract using a "
                                 "regular expression"),
                        cl::ZeroOrMore, cl::value_desc("rfunction"),
                        cl::cat(ExtractCat));
 
 // ExtractBlocks - The blocks to extract from the module.
 static cl::list<std::string> ExtractBlocks(
-    "bb", cl::desc("Specify <function, basic block> pairs to extract"),
-    cl::ZeroOrMore, cl::value_desc("function:bb"), cl::cat(ExtractCat));
+    "bb",
+    cl::desc("Specify <function, basic block[;group]> pairs to extract, "
+             "region must be single entry."),
+    cl::ZeroOrMore, cl::value_desc("function:bb[;bb2...]"),
+    cl::cat(ExtractCat));
 
 // ExtractAlias - The alias to extract from the module.
 static cl::list<std::string>
     ExtractAliases("alias", cl::desc("Specify alias to extract"),
                    cl::ZeroOrMore, cl::value_desc("alias"),
                    cl::cat(ExtractCat));
 
 // ExtractRegExpAliases - The aliases, matched via regular expression, to
 // extract from the module.
 static cl::list<std::string>
     ExtractRegExpAliases("ralias",