gccrs: Fix match-expression code-gen

gccrs: Fix match-expression code-gen

We were massing the match scruitinee expression as a way to access the
result of the expression. This is wrong and needs to be stored in a
temporary otherwise it will cause the code to be regnerated for each time
it is used. This is not an issue in the case where the expression is only
used once.

Fixes #1895

gcc/rust/ChangeLog:

	* backend/rust-compile-expr.cc (CompileExpr::visit): use a temp for the value

gcc/testsuite/ChangeLog:

	* rust/execute/torture/iter1.rs: New test.

Signed-off-by: Philip Herron <herron.philip@googlemail.com>