Add all the assignment code.

Signed-off-by: jmug <u.g.a.mariano@gmail.com>

hw4/README

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+Using main.native:
+
+main.native acts like the clang compiler.  Given several .ll, .c, and .o files,
+it will compile the .ll files to .s files (using the Compiler Design backend)
+and then combine the results with the .c and .o files to produce an executable
+named a.out.  You can also compile the .ll files using clang instead of the
+Compiler Design backend, which can be useful for testing purposes.
+
+
+* To run the automated test harness do:
+  ./main.native --test              
+
+* To compile ll files using the Compiler Design backend:
+  ./main.native path/to/foo.ll 
+
+  - creates output/foo.s   backend assembly code
+  - creates output/foo.o   assembled object file
+  - creates a.out          linked executable
+
+ NOTE: by default the .s and .o files are created in 
+ a directory called output, and the filenames are 
+ chosen so that multiple runs of the compiler will
+ not overwrite previous outputs.  foo.ll will be 
+ compiled first to foo.s then foo_1.s, foo_2.s, etc.
+
+
+* To compile ll files using the clang backend:
+  ./main.native --clang path/to/foo.ll
+
+* Useful flags:
+  --print-oat
+    pretty prints the Oat abstract syntax to the terminal
+
+  --print-ll 
+    echoes the ll program to the terminal
+
+  --print-x86
+    echoes the resulting .s file to the terminal
+
+  --interpret-ll
+    runs the ll file through the reference interpreter
+    and outputs the results to the console
+
+  --execute-x86
+    runs the resulting a.out file natively
+    (applies to either the Compiler Design backend or clang-compiled code)
+
+  --clang compiles to assembly using clang, not the Compiler Design backend
+
+  -v
+    generates verbose output, showing which commands are used
+    for linking, etc.
+
+  -op <dirname>
+    change the output path [DEFAULT=output]
+
+  -o 
+    change the generated executable's name [DEFAULT=a.out]
+
+  -S
+    stop after generating .s files 
+
+  -c 
+    stop after generating .o files 
+
+  -h or --help
+    display the list of options
+
+* Example uses:
+
+Run the test case /programs/factrect.ll using the Compiler Design backend:
+
+
+./main.native --execute-x86 programs/factrect.ll 
+--------------------------------------------------------------- Executing: a.out
+* a.out returned 120