Implement optimizeCompareInstr for PPC

Many PPC instructions have a so-called 'record form' which stores to a specific
condition register the result of comparing the result of the instruction with
zero (always as a signed comparison). For integer operations on PPC64, this is
always a 64-bit comparison.

This implementation is derived from the implementation in the ARM backend;
there are some differences because PPC condition registers are allocatable
virtual registers (although the record forms always use a specific one), and we
look for a matching subtraction instruction after the compare (but before the
first use) in addition to before it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179802 91177308-0d34-0410-b5e6-96231b3b80d8
[asm parser] Add support for predicating MnemonicAlias based on the assembler
variant/dialect.  Addresses a FIXME in the emitMnemonicAliases function.
Use and test case to come shortly.
rdar://13688439 and part of PR13340.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179804 91177308-0d34-0410-b5e6-96231b3b80d8
Disable PPC comparison optimization by default

This seems to cause a stage-2 LLVM compile failure (by crashing TableGen); do
I'm disabling this for now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179807 91177308-0d34-0410-b5e6-96231b3b80d8
Set the compact unwind encoding to 'requires EH DWARF' if we cannot generate a CU encoding.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179808 91177308-0d34-0410-b5e6-96231b3b80d8
Assert if we're trying to generate a null compact unwind entry.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179809 91177308-0d34-0410-b5e6-96231b3b80d8