Add isRegSequence property.

This patch adds a new property: isRegSequence and the related target hooks:
TargetIntrInfo::getRegSequenceInputs and
TargetInstrInfo::getRegSequenceLikeInputs to specify that a target specific
instruction is a (kind of) REG_SEQUENCE.

<rdar://problem/12702965>
R600/SI: Fix broken test
R600/SI: Clear lds bit on MUBUF instructions used for private stores

This bit was left uninitialized, which was causing some random failures
of piglit tests.

NOTE: This is a candidate for the 3.5 branch.
R600/SI: Add check for low 32 bits of encoding to mubuf tests

There are no variable values like registers encoded in the low 32 bits of MUBUF
instructions, so it is relatively easy to check these bits, and it will
help prevent us from introducing encoding bugs.
R600/SI: Add an _OFFEN variant MUBUF_STORE_* and use it for scratch writes
R600/SI: Add a ComplexPattern for selecting MUBUF _OFFSET variant

This saves us from having to copy a 64-bit 0 value into VGPRs for
BUFFER_* instruction which only have a 12-bit immediate offset.