arm: Fix ICE due to infinite splitting [PR109800]

arm: Fix ICE due to infinite splitting [PR109800]

In r11-966-g9a182ef9ee011935d827ab5c6c9a7cd8e22257d8 we introduce a
simplification to emit_move_insn that attempts to simplify moves of the form:

(set (subreg:M1 (reg:M2 ...)) (constant C))

where M1 and M2 are of equal mode size. That is problematic for the splitter
vfp.md:no_literal_pool_df_immediate in the arm backend, which tries to pun an
lvalue DFmode pseudo into DImode and assign a constant to it with
emit_move_insn, as the new transformation simply undoes this, and we end up
splitting indefinitely.

This patch changes things around in the arm backend so that we use a
DImode temporary (instead of DFmode) and first load the DImode constant
into the pseudo, and then pun the pseudo into DFmode as an rvalue in a
reg -> reg move. I believe this should be semantically equivalent but
avoids the pathalogical behaviour seen in the PR.

gcc/ChangeLog:

	PR target/109800
	* config/arm/arm.md (movdf): Generate temporary pseudo in DImode
	instead of DFmode.
	* config/arm/vfp.md (no_literal_pool_df_immediate): Rather than punning an
	lvalue DFmode pseudo into DImode, use a DImode pseudo and pun it into
	DFmode as an rvalue.

gcc/testsuite/ChangeLog:

	PR target/109800
	* gcc.target/arm/pure-code/pr109800.c: New test.