[ASan] Don't use stack malloc for 32-bit functions using inline asm

This prevents us from running out of registers in the backend.

Introducing stack malloc calls prevents the backend from recognizing the
inline asm operands as stack objects. When the backend recognizes a
stack object, it doesn't need to materialize the address of the memory
in a physical register. Instead it generates a simple SP-based memory
operand. Introducing a stack malloc forces the backend to find a free
register for every memory operand. 32-bit x86 simply doesn't have enough
registers for this to succeed in most cases.

Reviewers: kcc, samsonov

Differential Revision: http://reviews.llvm.org/D8790

llvm-svn: 233979