BUILD_PAIR and EXTRACT_ELEMENT and endianness.

The descriptions of these two operations:

EXTRACT_ELEMENT - This is used to get the lower or upper (determined by a Constant, which is required to be operand #1) half of the integer or float value specified as operand #0. This is only for use before legalization, for values that will be broken into multiple registers.

BUILD_PAIR - This is the opposite of EXTRACT_ELEMENT in some ways. Given two values of the same integer value type, this produces a value twice as big. Like EXTRACT_ELEMENT, this can only be used before legalization.

This is ambiguous about whether the halves are in terms of significance or memory order.

This does not matter for LE machines, but it does for BE.