When doing int<->ptr coercion for big-endian, calculate the shift amount correctly.

Previously we calculated the shift amount based upon DataLayout::getTypeAllocSizeInBits.
This will only work for legal types - types such as i24 that are created as part of
structs for bitfields will return "32" from that function. Change to using
getTypeSizeInBits.

It turns out that AArch64 didn't run across this problem because it always returned
[1 x i64] as the type for a bitfield, whereas ARM64 returns i64 so goes down this
(better, but wrong) codepath.