[x86] promote 'add nsw' to a wider type to allow more combines

The motivation for this patch starts with PR20134:
https://llvm.org/bugs/show_bug.cgi?id=20134

void foo(int *a, int i) {
  a[i] = a[i+1] + a[i+2];
}

It seems better to produce this (14 bytes):

movslq	%esi, %rsi
movl	0x4(%rdi,%rsi,4), %eax
addl	0x8(%rdi,%rsi,4), %eax
movl	%eax, (%rdi,%rsi,4)

Rather than this (22 bytes):

leal	0x1(%rsi), %eax
cltq             
leal	0x2(%rsi), %ecx      
movslq	%ecx, %rcx     
movl	(%rdi,%rcx,4), %ecx
addl	(%rdi,%rax,4), %ecx
movslq	%esi, %rax       
movl	%ecx, (%rdi,%rax,4)

The most basic problem (the first test case in the patch combines constants) should also be fixed in InstCombine, 
but it gets more complicated after that because we need to consider architecture and micro-architecture. For
example, AArch64 may not see any benefit from the more general transform because the ISA solves the sexting in
hardware. Some x86 chips may not want to replace 2 ADD insts with 1 LEA, and there's an attribute for that: 
FeatureSlowLEA. But I suspect that doesn't go far enough or maybe it's not getting used when it should; I'm 
also not sure if FeatureSlowLEA should also mean "slow complex addressing mode".

I see no perf differences on test-suite with this change running on AMD Jaguar, but I see small code size
improvements when building clang and the LLVM tools with the patched compiler.

A more general solution to the sext(add nsw(x, C)) problem that works for multiple targets is available
in CodeGenPrepare, but it may take quite a bit more work to get that to fire on all of the test cases that
this patch takes care of.

Differential Revision: http://reviews.llvm.org/D13757

llvm-svn: 250560