This patch is another installment in the series of Nios II patches aimed 
at removing or isolating hard-wired instruction encodings.  This will 
make the code more extensible for the future addition of new instruction 
set revisions.

In this patch, all explicit matching of opcodes and masks in 
nios2_in_epilogue_p, nios2_analyze_prologue, and nios2_get_net_pc is 
eliminated in favor of invoking the disassembler (via 
nios2_find_opcode_hash) to do that part.  A new set of helper functions 
are introduced to extract instruction operands according to the format 
of the matched instruction opcode.  Future ISA extensions are likely to 
include multiple encodings of some logical operations, such as add or 
subtract, so this organization will allow those details to be 
consolidated in the new helper functions instead of handled inline at 
call sites for those functions.  Also, organizing the analysis by what 
the instructions being examined conceptually do instead of by their 
format and encoding makes it easier to understand.

This patch also fixes an outstanding bug in the code.  Formerly, there 
were assumptions that the prologue and epilogue could only include one 
stack adjustment each.  This used to be true of code emitted by GCC 
except for functions with stack frame too large to be addressed via a 
16-bit offset.  A change made to GCC earlier this year (r208472) to 
correct an ABI conformance issue also means that many functions with 
frame pointers now have an additional stack pointer adjustment too.  In 
lifting the restriction, I made the prologue analyzer a little smarter 
in differentiating valid prologue stack adjustments (decrementing the 
SP, setting the FP from the SP) from those that can only appear in 
epilogues (incrementing the SP, setting the SP from the FP).

Test results on nios2-elf look good.  OK to commit?

-Sandra