[asan] Prevent rematerialization of &__asan_shadow.

Summary:
In the mode when ASan shadow base is computed as the address of an
external global (__asan_shadow, currently on android/arm32 only),
regalloc prefers to rematerialize this value to save register spills.
Even in -Os. On arm32 it is rather expensive (2 loads + 1 constant
pool entry).

This changes adds an inline asm in the function prologue to suppress
this behavior. It reduces AsanTest binary size by 7%.

Reviewers: pcc, vitalybuka

Subscribers: aemerson, kristof.beyls, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D40048

llvm-svn: 318235