rewrite the memset_pattern pattern generation stuff to accept any 2/4/8/16-byte

rewrite the memset_pattern pattern generation stuff to accept any 2/4/8/16-byte
constant, including globals.  This makes us generate much more "pretty" pattern
globals as well because it doesn't break it down to an array of bytes all the
time.

This enables us to handle stores of relocatable globals.  This kicks in about
48 times in 254.gap, giving us stuff like this:

@.memset_pattern40 = internal constant [2 x %struct.TypHeader* (%struct.TypHeader*, %struct.TypHeader*)*] [%struct.TypHeader* (%struct.TypHeader*, %struct
.TypHeader*)* @IsFalse, %struct.TypHeader* (%struct.TypHeader*, %struct.TypHeader*)* @IsFalse], align 16

...
  call void @memset_pattern16(i8* %scevgep5859, i8* bitcast ([2 x %struct.TypHeader* (%struct.TypHeader*, %struct.TypHeader*)*]* @.memset_pattern40 to i8*
), i64 %tmp75) nounwind

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@126044 91177308-0d34-0410-b5e6-96231b3b80d8