Factor the symbol handling in normalizedToAtoms into a separate function.

No functionality change.
Add help text for -MMD, -MD, -MM, -M, -MF.

Also regroup these flags so that alike flags are next to each other, while
keeping the list still mostly alphabetical.

The help text isn't ideal, but I feel it's less maze-like than
http://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Preprocessor-Options.html (look
at the entry for '-MMD' and count how many other entries you need to look up
until you know what it does).

And it looks like -M / -MM are mostly an historical accident and most people
use -MD or -MMD for deps tracking these days, so make -M / -MM refer to
-MD / -MMD instead of the other way round.
correct target directive handling error handling

The target specific parser should return `false' if the target AsmParser handles
the directive, and `true' if the generic parser should handle the directive.
Many of the target specific directive handlers would `return Error' which does
not follow these semantics.  This change simply changes the target specific
routines to conform to the semantis of the ParseDirective correctly.

Conformance to the semantics improves diagnostics emitted for the invalid
directives.  X86 is taken as a sample to ensure that multiple diagnostics are
not presented for a single error.
[AArch64 NEON] Add more scenarios to use perm instructions when lowering shuffle_vector

This patch covered 2 more scenarios:

1.  Two operands of shuffle_vector are the same, like
%shuffle.i = shufflevector <8 x i8> %a, <8 x i8> %a, <8 x i32> <i32 0, i32 2, i32 4, i32 6, i32 8, i32 10, i32 12, i32 14>

2. One of operands is undef, like
%shuffle.i = shufflevector <8 x i8> %a, <8 x i8> undef, <8 x i32> <i32 0, i32 2, i32 4, i32 6, i32 8, i32 10, i32 12, i32 14>

After this patch, perm instructions will have chance to be emitted instead of lots of INS.
[AArch64 NEON] Add missing patterns for bitcast from or to v1f64