[AArch64] Use 16 bytes as preferred function alignment on Cortex-A57.

[fhahn]

Summary

This change gives a 0.89% speed on execution time, a 0.94% improvement
in benchmark scores and a 0.62% increase in binary size on a Cortex-A57.
These numbers are the geomean results on a wide range of benchmarks from
the test-suite, SPEC2000, SPEC2006 and a range of proprietary suites.

The software optimization guide for the Cortex-A57 recommends 16 byte
branch alignment.

[mcrosier]

From a coding standpoint this all LGTM. However, I'm going to defer to a A57 code owner for final approval.

[kristof.beyls]

LGTM from a Cortex-A57 point-of-view: this patch is generating code in line with the optimization guide recommendations and the benchmark numbers quoted look good.

[fhahn]

rebased

lib/Target/AArch64/AArch64Subtarget.cpp

void AArch64Subtarget::initializeProperties() {
   case Cyclone:
     CacheLineSize = 64;
     PrefetchDistance = 280;
     MinPrefetchStride = 2048;
     MaxPrefetchIterationsAhead = 3;
     break;
   case CortexA57:
     MaxInterleaveFactor = 4;
+    PrefFunctionAlignment = 4;
     break;
   case ExynosM1:
     MaxInterleaveFactor = 4;
     MaxJumpTableSize = 8;
     PrefFunctionAlignment = 4;
     PrefLoopAlignment = 3;
     break;
   case Falkor:

test/CodeGen/AArch64/preferred-function-alignment.ll

 ; RUN: llc -mtriple=aarch64-unknown-linux -mcpu=generic < %s | FileCheck --check-prefix=ALIGN2 %s
 ; RUN: llc -mtriple=aarch64-unknown-linux -mcpu=thunderx < %s | FileCheck --check-prefix=ALIGN3 %s
 ; RUN: llc -mtriple=aarch64-unknown-linux -mcpu=thunderxt81 < %s | FileCheck --check-prefix=ALIGN3 %s
 ; RUN: llc -mtriple=aarch64-unknown-linux -mcpu=thunderxt83 < %s | FileCheck --check-prefix=ALIGN3 %s
 ; RUN: llc -mtriple=aarch64-unknown-linux -mcpu=thunderxt88 < %s | FileCheck --check-prefix=ALIGN3 %s
 ; RUN: llc -mtriple=aarch64-unknown-linux -mcpu=thunderx2t99 < %s | FileCheck --check-prefix=ALIGN3 %s
+; RUN: llc -mtriple=aarch64-unknown-linux -mcpu=cortex-a57 < %s | FileCheck --check-prefix=ALIGN4 %s
 ; RUN: llc -mtriple=aarch64-unknown-linux -mcpu=exynos-m1 < %s | FileCheck --check-prefix=ALIGN4 %s
 
 define void @test() {
   ret void
 }
 
 ; CHECK-LABEL: test
 ; ALIGN2: .p2align 2
 ; ALIGN3: .p2align 3
 ; ALIGN4: .p2align 4