ARM Ld/St Optimizer fix.

Allow LDRD to be formed from pairs with different LDR encodings. This was the original intention of the pass. Somewhere along the way, the LDR opcodes were refined which broke the optimization. We really don't care what the original opcodes are as long as they both map to the same LDRD and the immediate still fits.

Fixes rdar://10435045 ARMLoadStoreOptimization cannot handle mixed LDRi8/LDRi12

llvm-svn: 147922