Fix PR20056: Implement pseudo LDR <reg>, =<literal/label> for AArch64

This patch is based on the changes from ARM target [1,2]

Based on ARM doc [3], if the literal value can be loaded with a valid MOV,
it can emit that instruction. This is implemented in this patch.

[1] Fix PR18345: ldr= pseudo instruction produces incorrect code when using in inline assembly
Author: David Peixotto <dpeixott@codeaurora.org>
commit b92cca222898d87bbc764fa22e805adb04ef7f13 (r200777)
[2] Implement the ldr-pseudo opcode for ARM assembly
Author: David Peixotto <dpeixott@codeaurora.org>
commit 0fa193b08627927ccaa0804a34d80480894614b8 (r197708)
[3] http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0802a/CJAHAIBC.html

Differential Revision: http://reviews.llvm.org/D4163