[arm] Early split zero- and sign-extension

[arm] Early split zero- and sign-extension

This patch changes the insn patterns for zero- and sign-extend into
define_expands that generate the appropriate word operations
immediately.

	* config/arm/arm.md (zero_extend<mode>di2): Convert to define_expand.
	(extend<mode>di2): Likewise.

From-SVN: r277166