[AArch64][GlobalISel] Enable GlobalISel at -O0 by default

Tests updated to explicitly use fast-isel at -O0 instead of implicitly.

This change also allows an explicit -fast-isel option to override an
implicitly enabled global-isel. Otherwise -fast-isel would have no effect at -O0.

Differential Revision: https://reviews.llvm.org/D41362

llvm-svn: 321655
diff --git a/llvm/test/CodeGen/AArch64/arm64-fast-isel-conversion-fallback.ll b/llvm/test/CodeGen/AArch64/arm64-fast-isel-conversion-fallback.ll
index 16a02de..b3e649c 100644
--- a/llvm/test/CodeGen/AArch64/arm64-fast-isel-conversion-fallback.ll
+++ b/llvm/test/CodeGen/AArch64/arm64-fast-isel-conversion-fallback.ll
@@ -1,4 +1,4 @@
-; RUN: llc -O0 -verify-machineinstrs -mtriple=arm64-eabi < %s | FileCheck --enable-var-scope %s
+; RUN: llc -O0 -fast-isel -verify-machineinstrs -mtriple=arm64-eabi < %s | FileCheck --enable-var-scope %s
 
 ; Test fptosi
 define i32 @fptosi_wh(half %a) nounwind ssp {