[ms-inline-asm] Fixing bug in single asm statement support

Fixing a crash caused by trying to merge a single-line asm statement with an asm block that follows it, e.g:
asm int 4
asm {
  int 5
}

Now, only adjacent single-line asm statements that are not surrounded by braces will be merged into one asm call.

Differential Revision: http://reviews.llvm.org/D17496

llvm-svn: 261618