PR target/115751: Avoid force_reg in ix86_expand_ternlog.

This patch fixes a problem with splitting of complex AVX512 ternlog
instructions on x86_64.  A recent change allows the ternlog pattern
to have multiple mem-like operands prior to reload, by emitting any
"reloads" as necessary during split1, before register allocation.
The issue is that this code calls force_reg to place the mem-like
operand into a register, but unfortunately the vec_duplicate (broadcast)
form of operands supported by ternlog isn't considered a "general_operand",
i.e. supported by all instructions.  This mismatch triggers an ICE in
the middle-end's force_reg, even though the x86 supports loading these
vec_duplicate operands into a vector register in a single (move)
instruction.

This patch resolves this problem by replacing force_reg with calls
to gen_reg_rtx and emit_move (as the i386 backend, unlike the middle-end,
knows these will be recognized by recog).

2024-07-06  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR target/115751
	* config/i386/i386-expand.cc (ix86_expand_ternlog): Avoid use of
	force_reg to "reload" non-register operands, as these may contain
	vec_duplicate (broadcast) operands that aren't supported by
	force_reg.  Use (safer) gen_reg_rtx and emit_move instead.