[inlineasm] Attach readonly and readnone to inline-asm instructions.
Previously, clang/llvm treated inline-asm instructions conservatively,
choosing not to eliminate the instructions or hoisting them out of a loop
even when it was safe to do so. This commit makes changes to attach a
readonly or readnone attribute to an inline-asm instruction, which enables
passes such as LICM and EarlyCSE to move or optimize away the instruction.
Differential Revision: http://reviews.llvm.org/D10546