Enhance the init generation logic to emit a memset followed by a few stores when

Enhance the init generation logic to emit a memset followed by a few stores when
a global is larger than 32 bytes and has fewer than 6 non-zero values in the
initializer.  Previously we'd turn something like this:

char test8(int X) {
  char str[10000] = "abc";

into a 10K global variable which we then memcpy'd from.  Now we generate:

  %str = alloca [10000 x i8], align 16
  %tmp = getelementptr inbounds [10000 x i8]* %str, i64 0, i64 0
  call void @llvm.memset.p0i8.i64(i8* %tmp, i8 0, i64 10000, i32 16, i1 false)
  store i8 97, i8* %tmp, align 16
  %0 = getelementptr [10000 x i8]* %str, i64 0, i64 1
  store i8 98, i8* %0, align 1
  %1 = getelementptr [10000 x i8]* %str, i64 0, i64 2
  store i8 99, i8* %1, align 2

Which is much smaller in space and also likely faster.

This is part of PR279

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@120645 91177308-0d34-0410-b5e6-96231b3b80d8