PNaCl ABI: Disallow built-in multiplication in "alloca" instructions

Simplify the set of "alloca" instructions the ABI verifier allows.

Before this change, we used i8 arrays, such as:

  alloca [8 x i8]

After this change, we will just use i8 with an explicit size value, so
that becomes:

  alloca i8, i32 8

Allocation of variable-length arrays will require an explicit multiply
instruction.  This means that the code generator no longer has to
handle an implicit multiplication in "alloca", reducing the burden on
fast-and-simple code generators a little.  This means the PNaCl ABI
doesn't need to specify whether alloca's implicit multiplication
checks for overflow.

This doesn't affect what the backend generates.  See
lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp, which handles
constant-size AllocaInsts (by filling out StaticAllocaMap) and which
is uses for both -O2 (SelectionDAG) and -O0 (FastISel) translation.

BUG=https://code.google.com/p/nativeclient/issues/detail?id=3343
TEST=*.ll tests + PNaCl toolchain trybots

Review URL: https://codereview.chromium.org/17631004

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